Add the ability to dynamicaly register MediaPlayer types.

Add the ability to dynamically register low level MediaPlayer
factories which will be probed at setDataSource time to determine the
proper MediaPlayerBase to instantiate.

This change is in preparation for moving libaah_rtp out of
frameworks/base and into phantasm platform directory.

Change-Id: Icf8904db3ab9e3c85df6e780d5546d9988cb9076
Signed-off-by: John Grossman <johngro@google.com>