Improve ACTION_PACKAGE_CHANGED handling in IMMS

This CL fixes a false negative case when handling
ACTION_PACKAGE_CHANGED in IMMS#MyPackageMonitor with greatly reducing
unnecessary false positives cases as well.

PackageMonitor#onPackageChanged(), which is the default handler of
ACTION_PACKAGE_CHANGED, returns true when and only when the entire
package state is changed to let InputMethodManagerService (IMMS)
rebuild the list of available IMEs when it returns true. Here we have
a false negative case and false positive cases.

Case 1 - false negative (Bug 28181208)

  If ACTION_PACKAGE_CHANGED was about some components not the
  entire package itself, currently MyPackageMonitor#onPackageChanged()
  returns false and IMMS fails to rebuild the list of available IMEs.

Case 2 - false positive (contributing to Bug 32343335)

  Even if ACTION_PACKAGE_CHANGED was about a package that implements
  no IME service at all, currently MyPackageMonitor#onPackageChanged()
  returns true and IMMS ends up with rebuilding the list of avilable
  IMEs unnecessarily. Note that package replacement is a different
  story that should be dealt with ACTION_PACKAGE_{ADDED, REMOVED}.

For both cases, luckily we can easily ask PackageManager to give the
list of relevant package names that might contain IMEs regardless of
enabled/disabled state, which is exactly what we want to use the watch
list for ACTION_PACKAGE_CHANGED events.

For the case 3, we can just check the current user ID.

Test: Manually verified as follows.
       1. adb root
       2. adb install -r LatinIME.apk
       3. adb shell dumpsys input_method
           Make sure that com.android.inputmethod.latin/.LatinIME is
           recognized by IMMS.
       4. adb shell pm disable com.android.inputmethod.latin/.LatinIME
       5. adb shell dumpsys input_method
           Make sure that com.android.inputmethod.latin/.LatinIME is
           no longer recognized by IMMS.
       6. adb shell pm enable com.android.inputmethod.latin/.LatinIME
       7. adb shell dumpsys input_method
           Make sure that com.android.inputmethod.latin/.LatinIME is
           recognized by IMMS again.
Test: Manually verified as follows.
       1. Build a custom APK LatinIME_no_ime.apk that has no input
          method service.
       2. adb install -r LatinIME_no_ime.apk
       3. adb shell dumpsys input_method
           Make sure that com.android.inputmethod.latin/.LatinIME is
           not recognized by IMMS.
       4. adb install -r LatinIME.apk
       5. adb shell dumpsys input_method
           Make sure that com.android.inputmethod.latin/.LatinIME is
           recognized by IMMS.
       6. adb install -r LatinIME_no_ime.apk
       7. adb shell dumpsys input_method
           Make sure that com.android.inputmethod.latin/.LatinIME is
           no longer recognized by IMMS.
Bug: 32343335
Fixes: 28181208
Change-Id: I7b69c349318ce06a48d03a4468cf2c45bfb73dc2