libstagefright: Added mime-types for enca/encv atoms

- enca/encv are encrypted audio and video atoms for
  mp4a and mp4v. Hence returning aac and mpeg4 mime-types
  for enca and encv.

CRs-Fixed: 578506

Change-Id: Ie568ea7eec73e110eef07e2b1b921afefc8c54a7