Add accessibility actions for text editing.

Currently text editing is pretty hard (certain operations even
impossible) for a blind person. To address the issue this change
adds APIs that enable an accessibility service to perform basic
text editing operations such as copy, paste, cut, set selection,
extend selection while moving at a given granularity.

The new APIs enable an accessibility service to expose a gesture
driven efficient text editing facility.

bug:8098384

Change-Id: I82b200138a3fdf4c0c316b774fc08a096ced29d0