Adding an announcement type accessibility event and a method on View to announce.

1. The need for sending an accessibility event to announce a context change
   which does not cleanly fit into the existing UI transition UI events has
   come quite a few time in application development. To avoid retrofitting
   accessibility event types that do not semantically match the intent to
   just announce a short message this patch is adding specialized event type.
   Also a helper method on View is added to sheild developers from knowing
   how to construct and send such an event.

bug:5977979

Change-Id: Iaf5f620426f8616be67fbf243a02ad5b606c949b