Add MEDIA_STARTED/PAUSED/STOPPED events to media players

This is needed for the MediaTimeProvider java interface, so it does not
continually poll for current media time.

Note: NuPlayer and AwesomePlayer do not correctly handle stop (pause
instead), so for those we will signal PAUSED.

Signed-off-by: Lajos Molnar <lajos@google.com>
Change-Id: I3c61e1bda475f131323f475c18a42e3ec66c9ae1
Bug: 10326117