DO NOT MERGE 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 along with I1d60e529bedc56a60b06fb4760852955543e2b7b is in
preparation for moving libaah_rtp out of frameworks/base and into
phantasm platform directory.

Signed-off-by: John Grossman <johngro@google.com>
Change-Id: I54f62956d12359a237245deefd76756f196a8fa3