MediaSessionService: Cache active sessions per an user

This fixes an issue that MediaSessionManager.getActiveSessions() may
return media sessions of the wrong user. This had been an issue since
the Android M.

Bug: 36227938
Test: Run the CtsMediaTest (MediaSessionTest and MediaSessionManagerTest)
  and CtsMediaHostTestCases
Change-Id: I8a8297e4d182f52c84f654bdb0ba78bc16a1c058