Migrate TextView to use OnBackInvokedDispatcher.

The OnBackInvokedCallback will be called when an app is requesting the new back dispatch behavior. Otherwise we keep dispatching KEYCODE_BACK to TextViews.

Test: Use backTestApp WidgetTestsActivity. Try selecting texts and
dimisss selection by swiping back. Both on TextView and EditText and
AutoCompleteTextView.

Change-Id: I9422f28ab650c5d77c75394fe6dcf2a3981f79c7