Replace generic OnEvent with actual event callbacks

Also pass session_id and event_listener to PolicyEngine to make it easier
to dispatch events from PolicyEngine.

Bug: 19771437

Merged from Widevine CDM repo:
https://widevine-internal-review.googlesource.com/#/c/13816/

Change-Id: I5723cb371cb3c43c945051af3402b09069ba5859

libwvdrmengine/cdm/core/test/license_unittest.cpp

@@ -106,7 +106,8 @@ class MockCryptoSession : public CryptoSession {
 
 class MockPolicyEngine : public PolicyEngine {
  public:
-  MockPolicyEngine(CryptoSession* crypto) : PolicyEngine(crypto) {}
+  MockPolicyEngine(CryptoSession* crypto)
+      : PolicyEngine("mock_session_id", NULL, crypto) {}
 };
 
 class MockClock : public Clock {