Generate renewal and expiry events

The android timer class was not generating timer events correctly. This caused
renewal and expiration events not to be sent. A strong pointer to the
timer thread was not held and this caused the android util timer
thread to exit after firing once. This is now addressed.

Bug: 8736545

Merge of https://widevine-internal-review.googlesource.com/#/c/5353/
from the Widevine CDM repository.

Change-Id: I2d904e55d4d10eacc1a51f1c6b5c1a267c92c8d8

libwvdrmengine/cdm/core/src/cdm_engine.cpp

@@ -101,6 +101,8 @@ CdmResponseType CdmEngine::CloseSession(const CdmSessionId& session_id) {
   }
 
   sessions_.erase(session_id);
+  if (sessions_.size() == 0)
+    DisablePolicyTimer();
   iter->second->DestroySession();
   delete iter->second;
   return NO_ERROR;