Don't use metalava for "check-current"

Because we already generate the latest current.txt and removed.txt
during the "main" metalava invocation (i.e. stub generation),
we don't have to use metalava for current API check.

Just use diff instead.

Bug: 151160048

Test: Remove @hide from Intent.EXTRA_INSTALL_RESULT, and do `m out/soong/.intermediates/frameworks/base/api-stubs-docs/android_common/check_current_api.timestamp`.
Result is:
```
FAILED: out/soong/.intermediates/frameworks/base/api-stubs-docs/android_common/check_current_api.timestamp
( true && diff -u -F '{ *$' frameworks/base/api/current.txt out/soong/.intermediates/frameworks/base/api-stubs-docs/android_common/api-stubs-docs_api.txt && diff -u -F '{ *$' frameworks/base/api/removed.txt out/soong/.intermediates/frameworks/base/api-stubs-docs/android_common/api-stubs-docs_removed.txt && touch out/soong/.intermediates/frameworks/base/api-stubs-docs/android_common/check_current_api.timestamp ) || ( echo -e "\n******************************\nYou have tried to change the API from what has been previously approved.\n\nTo make these errors go away, you have two choices:\n   1. You can add '@hide' javadoc comments (and remove @SystemApi/@TestApi/etc)\n      to the new methods, etc. shown in the above diff.\n\n   2. You can update current.txt and/or removed.txt by executing the following command:\n         make api-stubs-docs-update-current-api\n\n      To submit the revised current.txt to the main Android repository,\n      you will need approval.\n******************************\n" ; exit 38 )

Change-Id: I63f237c6e046de8a6b6538c44ec0ab8bd8fbbdb1
--- frameworks/base/api/current.txt     2020-04-16 17:50:21.911899599 -0700
+++ out/soong/.intermediates/frameworks/base/api-stubs-docs/android_common/api-stubs-docs_api.txt       2020-04-16 17:51:35.211792638 -0700
@@ -10415,6 +10415,7 @@ public class Intent implements java.lang
     field public static final String EXTRA_INDEX = "android.intent.extra.INDEX";
     field public static final String EXTRA_INITIAL_INTENTS = "android.intent.extra.INITIAL_INTENTS";
     field public static final String EXTRA_INSTALLER_PACKAGE_NAME = "android.intent.extra.INSTALLER_PACKAGE_NAME";
+    field public static final String EXTRA_INSTALL_RESULT = "android.intent.extra.INSTALL_RESULT";
     field public static final String EXTRA_INTENT = "android.intent.extra.INTENT";
     field public static final String EXTRA_KEY_EVENT = "android.intent.extra.KEY_EVENT";
     field public static final String EXTRA_LOCAL_ONLY = "android.intent.extra.LOCAL_ONLY";

******************************
You have tried to change the API from what has been previously approved.

To make these errors go away, you have two choices:
   1. You can add '@hide' javadoc comments (and remove @SystemApi/@TestApi/etc)
      to the new methods, etc. shown in the above diff.

   2. You can update current.txt and/or removed.txt by executing the following command:
         make api-stubs-docs-update-current-api

      To submit the revised current.txt to the main Android repository,
      you will need approval.
******************************

ninja: build stopped: subcommand failed.
```

Test: Remove @removed and @SystemApi from Intent.ACTION_MASTER_CLEAR, and do `m out/soong/.intermediates/frameworks/base/system-api-stubs-docs/android_common/check_current_api.timestamp`
Result is:
```
( true && diff -u -F '{ *$' frameworks/base/api/system-current.txt out/soong/.intermediates/frameworks/base/system-api-stubs-docs/android_common/system-api-stubs-docs_api.txt && diff -u -F '{ *$' frameworks/base/api/system-removed.txt out/soong/.intermediates/frameworks/base/system-api-stubs-docs/android_common/system-api-stubs-docs_removed.txt && touch out/soong/.intermediates/frameworks/base/system-api-stubs-docs/android_common/check_current_api.timestamp ) || ( echo -e "\n******************************\nYou have tried to change the API from what has been previously approved.\n\nTo make these errors go away, you have two choices:\n   1. You can add '@hide' javadoc comments (and remove @SystemApi/@TestApi/etc)\n      to the new methods, etc. shown in the above diff.\n\n   2. You can update current.txt and/or removed.txt by executing the following command:\n         make system-api-stubs-docs-update-current-api\n\n      To submit the revised current.txt to the main Android repository,\n      you will need approval.\n******************************\n" ; exit 38 )
--- frameworks/base/api/system-removed.txt      2020-04-16 17:50:21.911899599 -0700
+++ out/soong/.intermediates/frameworks/base/system-api-stubs-docs/android_common/system-api-stubs-docs_removed.txt     2020-04-16 17:53:45.319602905 -0700
@@ -55,7 +55,6 @@ package android.content {

   public class Intent implements java.lang.Cloneable android.os.Parcelable {
     field @Deprecated public static final String ACTION_DEVICE_INITIALIZATION_WIZARD = "android.intent.action.DEVICE_INITIALIZATION_WIZARD";
-    field @Deprecated public static final String ACTION_MASTER_CLEAR = "android.intent.action.MASTER_CLEAR";
     field @Deprecated public static final String ACTION_SERVICE_STATE = "android.intent.action.SERVICE_STATE";
     field @Deprecated public static final String EXTRA_CDMA_DEFAULT_ROAMING_INDICATOR = "cdmaDefaultRoamingIndicator";
     field @Deprecated public static final String EXTRA_CDMA_ROAMING_INDICATOR = "cdmaRoamingIndicator";

******************************
You have tried to change the API from what has been previously approved.

To make these errors go away, you have two choices:
   1. You can add '@hide' javadoc comments (and remove @SystemApi/@TestApi/etc)
      to the new methods, etc. shown in the above diff.

   2. You can update current.txt and/or removed.txt by executing the following command:
         make system-api-stubs-docs-update-current-api

      To submit the revised current.txt to the main Android repository,
      you will need approval.
******************************
```

Test: Add `baseline_file` to `check_api.current` and run `m`
FAILED: out/soong/build.ninja
out/soong/.bootstrap/bin/soong_build -t -l out/.module_paths/Android.bp.list -b out/soong -n out -d out/soong/build.ninja.d -globFile out/soong/.bootstrap/build-globs.ninja -o out/soong/build.ninja Android.bp
error: frameworks/base/StubLibraries.bp:86:1: module "api-stubs-docs" variant "android_common": current API check can't have a baseline file. (module %s): api-stubs-docs
ninja: build stopped: subcommand failed.

Test: With the above changes, run "m api-stubs-docs-update-current-api system-api-stubs-docs-update-current-api", make sure the API files got updated, and then run "m"
Change-Id: I47b146c6fe4caa65775ecf5425ab09b57f43f839