MediaCas: add two sets of test plugins

- MockCasPlugin: does nothing other than API logging

- ClearKeyCasPlugin: implement a clear key CAS that carries content
keys in the clear in ECMs. It initializes from a test asset string
with a valid id. It outputs to clear buffers.

bug: 22804304
Change-Id: Ifc70b86c1ec80a45b75c7bdd1e558a59018ace5a