Avoid race condition on closing CDM session.

[ Merge from go/wvgerrit/22920 ]

Automated tests reveal a race condition between closing
a session and the periodic policy timer event. If a close
session was in progress (WVDrmPlugin::closeSession) and
CdmEngine::CloseSession caused the CdmEngine::session_list_lock_
to be held, a call into CdmEngine::OnTimerEvent would pend on the
release of the lock.

The close session would continue to deallocate the session
and disable (stop) the policy timer leaving the CdmEngine::OnTimerEvent
call in an undefined state. This would result in an ANR.

This subtle race-condition was introduced when changes were made
to add in per-origin storage [ http://go/wvgerrit/17971 ]. This seems
to happen at a low frequency (~ < 0.5%).

To address a lock has been introduced to protect the map
WvContentDecryptionModule::cdms_.

Test: Unit tests + 200 aupt test iterations

b/33343891

Change-Id: I9788db8a7d1df84f0df82cdbadb9d0f0fbe21e4e

libwvdrmengine/cdm/include/wv_content_decryption_module.h

@@ -148,6 +148,8 @@ class WvContentDecryptionModule : public android::RefBase, public TimerHandler {
   // instance variables
   // This manages the lifetime of the CDM instances.
   std::map<std::string, CdmInfo> cdms_;
+  Lock cdms_lock_;
+
   // This contains weak pointers to the CDM instances contained in |cdms_|.
   std::map<std::string, CdmEngine*> cdm_by_session_id_;