[automerger] Add tests to validate key length for clearkey plugin. am: c312e21b5e  -s ours am: 00b66950cb am: 319065b756 am: 7c98012dc8
am: e134b9dd23

Change-Id: I4ab4c97029a4f6d1de0a88c9e3a3c3d37849ea75