[Android] Bridge MediaPlayerBridge implementation to xwalk

Why: There are requirements to customize the MediaPlayer on Android
which is implemented in Chromium code base. Till now we have two
points: 1) Support the file:///android_asset in MediaPlayer; 2)
Support app scheme in the resource loading of MediaPlayer. To make
the modification and rebase efforts in Chromium less we use a
MediaPlayerBridge in xwalk instead, which contains the hack for xwalk.

How: Replace the MediaPlayerBridge.java in the JNI binding in Chromium
with a new one in xwalk. The MediaPlayerBridge in xwalk inherits the one
in Chromium with the same interface then we can override the points we
need.