[Android] Bridge MediaPlayerBridge implementation to xwalk

Add a new MediaPlayerBridge in xwalk around android.media.MediaPlayer
by inheriting org.chromium.media.MediaPlayerBridge, by which we can
customize the MediaPlayer in xwalk. Now we have two customized items:
 1. Support app scheme in the resource loading of MediaPlayer.
 2. Support file://android_asset/ url in MediaPlayer.