MediaSession: Add new methods for preparing media content

Introduced new methods for preparing media content: prepare,
prepareFromMediaId, prepareFromSearch, and prepareFromUri. These methods
allow MediaSession prepare media content rather than play
it. MediaSession will set up a queue or buffer their content during
these preparing requests.
The constants, FLAG_HANDLES_PREPARE_ONLY and EXTRA_PREPARE_ONLY, are
removed because the introduced methods can replace them.

Bug:25841735, Bug:25153060, Bug:25153353
Change-Id: I8fa2097c98fe2fba92b267d2865a9b75d118af09