Add a method to associate MediaDrm session with MediaCrypto

Previously, to associate a MediaDrm session with MediaCrypto,
the MediaDrm sessionId was passed as initData to the MediaCrypto
constructor.  This is not ideal for two reasons: it's pretty
obscure and you can't change the association without tearing
down the MediaCodec/MediaCrypto and starting all over.  Use
cases like key rotation require being able to update the
MediaDrm session post-construction.  This CL addresses both of
these issues.

bug: 19570317
Change-Id: Ieb32d04d61742fc1dee2105dd904690d44c46c7b