Fixes to DrmConvertedStatus, DrmInfoStatus, and DrmSupportInfo classes

o added missing javadoc comment for some public fields
o added checks for arguments
o DrmSupportInfo should have not been exposed as public.
  A default package access modifier appears to be more proper but we may
  not be able to change the modifier.

Change-Id: I5b284c17219c5121f241ee2934fb3e859ce7c827