Trigger newly-registered callbacks with events from currently active

translations.

Bug: 192331240
Test: atest android.translation.cts.UiTranslationManagerTest
Test: Manually on device:
  1. Kill Gboard
  2. Open translatable Chat conversation, wait for Live Translate
  4. Focus on chat input
  5. Auto-translate on Gboard is opened, and debug logs show
     "TranslationManagerServiceImpl: Triggering callback for..."

Change-Id: Ida82a947491e8157fac5cd43939d6d358fbe6c7b