Only allow valid types to be used in DrmErrorEvent and DrmInfoEvent

o This patch will prevent applications from abusing the API by using
  arbitrary int value for DrmErrorEvent or DrmInfoEvent types.
o We should have not defined some type constants in the super/base
  class, DrmEvent.

Change-Id: Id0bb12caa8ce471cb6951cc2b5d37ea408f90063