Playback duration should override license duration

[ Merge of go/wvgerrit/14360 ]

If within playback window, do not expire license on expiry of rental or
license duration. In this case playback duration will extend the license.

b/17791094

Change-Id: I26d255aa8f0287bd583ebdeec991c613d49d8f22

libwvdrmengine/cdm/core/include/policy_engine.h

@@ -83,6 +83,7 @@ class PolicyEngine {
                             int64_t last_playback_time);
 
   bool IsLicenseForFuture() { return license_state_ == kLicenseStatePending; }
+  bool IsPlaybackStarted() { return playback_start_time_ > 0; }
 
  private:
   friend class PolicyEngineTest;