MediaTimeProvider support fixes

Add MEDIA_SKIPPED event when resuming at a different time than
seeked to.

Send MEDIA_STARTED/PAUSED events only when playing (vs. when doing seek
previews)

Change-Id: I243ebf054303755ea8863229c3211694f2c204a7
Signed-off-by: Lajos Molnar <lajos@google.com>
Bug: 10954008