Implements the optional setMediaDrmSession() method. To enble this,
support was added to the core to report if a session ID is valid.
As a consequence of this, in the tests for the CryptoPlugin,
construction of the plugin must be deferred until all gMock
expectations are set, as construction now calls into the CDM core.
This is a merge of two changes from the Widevine CDM repo:
http://go/wvgerrit/14083
Allow Setting of Session ID
http://go/wvgerrit/14085
Check If Session ID Is Valid When Changing CryptoPlugin IDs
Bug: 19570317
Change-Id: I7dbd777ce6efebd71fdb5e602663a0e35a48a9c4
c5f576585b