[ Merge from http://go/wvgerrit/27261/ ]
Sessions created to release keys are periodically cleaned up if the
key release operation does not complete within a specific
amount of time. If other sessions are open, they will be released
through the timer thread. This would result in deadlock as a mutex
was taken twice.
Test: Verified by cdm_extended_duration_test
(AutomatedOfflineSessionReleaseOnOpenSession and
AutomatedOfflineSessionReleaseOnTimerEvent tests)
b/37546078
Change-Id: I7d45f939bdce77e5db461a401364da4f42c1c034