Allow Setting of Session ID

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

libwvdrmengine/cdm/core/test/cdm_engine_test.cpp

@@ -57,6 +57,7 @@ class WvCdmEngineTest : public testing::Test {
     }
     ASSERT_EQ(NO_ERROR, status);
     ASSERT_NE("", session_id_) << "Could not open CDM session.";
+    ASSERT_TRUE(cdm_engine_.IsOpenSession(session_id_));
   }
 
   virtual void TearDown() { cdm_engine_.CloseSession(session_id_); }