Detect when unable to meet policy requirements

[ Merge of http://go/wvgerrit/25781 ]

The security level (software/hardware, decryption/decode)
in the policy that specified how the key was to be used was
not being respected for L3. Playback would either continue or
a vendor specific error would be thrown.

If the device cannot use the key as permitted by the policy
CryptoException#ERROR_INSUFFICIENT_OUTPUT_PROTECTION will be thrown.

Test: Verified by WV unit+integration tests.
      Verified by WidevineDashPolicyTests
      Verified by WidevineDashPolicyTests#testL3SoftwareSecureDecoderRequired,
      testL3HardwareSecureCryptoRequired, testL3HardwareSecureDecodeRequired,
      testL3SecureVideoPathRequired.

b/31913737
b/31913439

Change-Id: Ibfc7f3dd6fc7264e8cf9b0d33f6f8d619eed6c00

libwvdrmengine/cdm/core/include/policy_engine.h

@@ -34,6 +34,10 @@ class PolicyEngine {
   // status is not calculated to avoid overhead in the decryption path.
   virtual bool CanDecryptContent(const KeyId& key_id);
 
+  // Verifies whether the policy allows use of the specified key of
+  // a given security level for content decryption.
+  virtual bool CanUseKey(const KeyId& key_id, CdmSecurityLevel security_level);
+
   // OnTimerEvent is called when a timer fires. It notifies the Policy Engine
   // that the timer has fired and dispatches the relevant events through
   // |event_listener_|.