Broadcast a content change event when MTP devices are added and removed.

Change-Id: Id6dc5607b3dacf292c5ec06d1ed447481a141f21
Signed-off-by: Mike Lockwood <lockwood@android.com>
Merge commit '2a5f03655d336076d07c5150402699fceb21e942' into HEAD

* commit '2a5f03655d336076d07c5150402699fceb21e942':
  Broadcast a content change event when MTP devices are added and removed.
Scan any external media for content

Change-Id: Ibbebce1d6c95472b07ae92d3355e5a60b5b7b45c