Reset crypto session pointers on RemoveKeys.

[ Merge of http://go/wvgerrit/189650 ]

The CDM session shares its CryptoSession instance with a few additional
member objects (CdmLicense and PolicyEngine).  When the CDM session's
crypto session is reset, it must also reset the CdmLicense and
PolicyEngine otherwise, a potential stale pointer reference may occur.

Test: request_license_test on Oriole
Test: WVTS on Oriole
Bug: 311239278
Change-Id: Ie175513ae652dcd96e12e5e1def574a8a56d5863

libwvdrmengine/cdm/core/include/policy_engine.h

@@ -106,6 +106,8 @@ class PolicyEngine {
 
   virtual const LicenseIdentification& license_id() { return license_id_; }
 
+  WvCdmEventListener* event_listener() { return event_listener_; }
+
   bool GetSecondsSinceStarted(int64_t* seconds_since_started);
   bool GetSecondsSinceLastPlayed(int64_t* seconds_since_started);
 
@@ -131,6 +133,11 @@ class PolicyEngine {
     return license_keys_->MeetsConstraints(key_id);
   }
 
+  // Testing only.  Caller retains ownership.
+  void set_crypto_session(CryptoSession* crypto_session) {
+    crypto_session_ = crypto_session;
+  }
+
  private:
   friend class PolicyEngineTest;
   friend class PolicyEngineConstraintsTest;