Update usages of min_sdk_version that relies on (kind+level)

The type of min_sdk_version is being migrated from
android.SdkSpec(kind+level) to android.ApiLevel(level). This affects the
{min|target}-sdk-version aapt2 flag for vendor modules. For these
modules, if min_sdk_version is not set in Android.bp files,
MinSdkVersion would default to SdkVersion, and therefore would get
access to the sdkKind it was compiling against.

To accomodate the upcoming change of min_sdk_version to ApiLevel,
explicitly use SdkVersion to determine the `min-sdk-version` flags for
modules targeting `current`.

Bug: 208456999
Test: no change in ninja files (this should be a noop)
Change-Id: Ie2924698687597cdd572d4bb4c687d0b1569c4b8