Unhide the android.media.[Media]DataSource interface.

This allows apps to implement MediaDataSource, which is modeled on
stagefright's DataSource, to supply media data to the framework. This
was already implemented for MediaExtractor, but it was renamed from
DataSource.

MediaExtractor, MediaPlayer and MediaMetadataRetriever each have a new
overload: #setDataSource(android.media.MediaDataSource)

Only NuPlayer supports this new data source.

The change introduces:
 * IDataSource:        The binder interface for DataSource.
 * JMediaDataSource:   The native counterpart to the java interface. It
                       implements IDataSource.
 * CallbackDataSource: A stagefright DataSource that wraps an IDataSource.

Change-Id: I6d9c1167b4a7384c469b1e928f31791a7ebed716