Auto finish composing text when A11yIME commits text

We have intentionally not exposed "composition" related APIs to
A11yIME to minimize the problem space of concurrent text editing
between the real IME and A11yIMEs.  To make this model more
predictable and reliable, it is planned to finish any existing
composing text when

  AccessibilityInputConnection#commitText()

gets called.  This CL actually make it happen.

Fix: 218749467
Test: Manually verified with sample A11yIme as follows.
  1. Build aosp_coral-userdebug and flash it.
  2. make -j EditTextVariations
  3. adb install -r $ANDROID_TARGET_OUT_TESTCASES/EditTextVariations/arm64/EditTextVariations.apk
  4. make -j SampleInputMethodAccessibilityService
  5. adb install -r $OUT/system/app/SampleInputMethodAccessibilityService/SampleInputMethodAccessibilityService.apk
  6. adb shell settings put secure enabled_accessibility_services com.example.android.sampleinputmethodaccessibilityservice/com.example.android.sampleinputmethodaccessibilityservice.SampleInputMethodAccessibilityService
  7. adb shell am start -n com.android.inputmethod.tools.edittextvariations/.EditTextVariations
  8. Tap the top edit field.
  9. Make sure that AOSP Keyboard shows up.
 10. Type "abc" with AOSP Keyboard.
 11. Hit "A" button on the sample A11yIME.
 12. Make sure that the text becomes "abcA".
Change-Id: I4afd279d8bc1a04d4e52ccb947dc28aed8f891f0