(This is a merge of https://widevine-internal-review.googlesource.com/#/c/11405 from the Widevine CDM Repo.) AUPT is revealing a crash when destructing WVDrmPlugin due to multi-threaded contention over the session map. As a fix, we are now protecting access to the map via a mutex. Bug: 17761616 Change-Id: Iddeca657effd3c7f3ff35ce334d7979291667cef