Fix race condition for cas sessions -- DO NOT MERGE

Change the session to shared_ptr and use atomic_load/store.

Test: POC; CTS MediaCasTest; CTS MediaDrmClearkeyTest#
testClearKeyPlaybackMpeg2ts
bug: 113027383

Change-Id: I75f4cb33a022f28d45918442d64c5c46df2640ef
(cherry picked from commit c927af1287e3b90d77e40aae26f4a3b61de3ac09)
4 files changed