Additions and cleanups for ImageDecoder API

Bug: 63909536
Test: CTS: I0f36ce34c968fd7fae4d8edebabea3a421859615

Add overloads for null listener, byte[] without offset + length
Clean up comments

Change-Id: I3dd1dae94cf1fe977d96fcae9b36cbed0adfe749
1 file changed