Use 0 to represent unexpired license in OnExpirationUpdate

Also fix a missing change for
"playback duration should override license duration".

Merged from Widevine CDM repo:
https://widevine-internal-review.googlesource.com/#/c/14435/

Bug: 21393975
Change-Id: Ibfcf3ae4c13db8944ea285bcc79b6312ea621e1b

libwvdrmengine/cdm/core/src/cdm_session.cpp

@@ -348,8 +348,7 @@ CdmResponseType CdmSession::Decrypt(const CdmDecryptionParameters& params) {
   } else {
     Clock clock;
     int64_t current_time = clock.GetCurrentTime();
-    if (policy_engine_->IsLicenseDurationExpired(current_time) ||
-        policy_engine_->IsPlaybackDurationExpired(current_time)) {
+    if (policy_engine_->IsLicenseOrPlaybackDurationExpired(current_time)) {
       return NEED_KEY;
     }
   }