Fix API issues in DrmRights class.

o account id and subscription id are not mandatory, and thus can be anything
  such as null or an empty string.
o removed unnecessary or thus inconsistent checks for account id and subscription id
o updated the javadoc to specify that mimeType could not be null or an empty string.

Change-Id: I704f1dd7fc5d33060da908aae0469d560c7db036