MediaCas: address comments for API reviews

- Wrap session id byte array in Session object

- Move session operations from MediaCas to Session

- Remove position prarameters on descramble() method

- Retrieve cas info for a track by getCasInfo() instead
  of getDrmInitData().

bug: 22804304
bug: 36791613
bug: 36783335

Change-Id: Ib3ad8d6a2f679c0e60d2bb025ac5999339722306