Update MediaPlayer to allow setVideoSurface calls after prepare. Also allow

passing a null surface. The API is now enabled to change the surface while
the video is playing. This could allow orientation changes during playback
or to allow the audio track from a video to play in the background.
NOTE: There are still changes required to pmem driver to allow remapping
shared physical memory into a process in order for this to work. This
change only enables the API to send the appropriate calls when the lower
level code supports it.