Make IInputMethodPrivilegedOperations to async (3/N)

-. Remove VoidResultCallback of reportFullscreenMode
   and let it be truly asynchronous.
-. Rename this method to reportFullscreenModeAsync.

Bug: 183587528
Test: atest CtsInputMethodTestCases
Test: Manually verified as follows.
   1. Build flame-userdebug and flash it.
   2. Make sure that the screen rotation is enabled.
   3. make -j SoftKeyboard
   4. adb install -r $OUT/system/app/SoftKeyboard/SoftKeyboard.apk
   5. adb shell ime enable com.example.android.softkeyboard/.SoftKeyboard
   6. adb shell ime set com.example.android.softkeyboard/.SoftKeyboard
   7. make -j EditTextVariations
   8. adb install -r $ANDROID_TARGET_OUT_TESTCASES/EditTextVariations/arm64/EditTextVariations.apk
   9. adb shell am start -n com.android.inputmethod.tools.edittextvariations/.EditTextVariations
  10. Make sure that the device is in the landscape mode,
      and the SoftKeyboard sample IME is not yet shown.
  11. adb shell dumpsys input_method | grep mFullscreenMode
      Then make sure the mFullscreenMode is "false"
  12. Tap the first edit field then make sure that SoftKeyboard
      sample IME becomes visible in the fullscreen mode.
  13. adb shell dumpsys input_method | grep mFullscreenMode
      Then make sure the mFullscreenMode is "true"
  14. Tap the down button on the navbar to hide the SoftKeyboard
      sample IME.
  15. adb shell dumpsys input_method | grep mFullscreenMode
      Then make sure the mFullscreenMode is "false"

Change-Id: I92e8b0d420be3dd16cc4f3ba29e0bde5f12ab2ce