Show and trigger activities that implement Text Processing actions

This CL adds the Activities that support Intent.ACTION_PROCESS_TEXT
to the Text Selection Action Mode in Editor, and triggers an intent
with the currently selected text when they are selected.

It also adds the required mechanism to allow a View to request an
intent to be started, and return the activity result back to it.

Change-Id: I62ec618010edf01da41338c8c1a7dd4292a15227