Fix race condition for cas sessions

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 7934a8f7ee0988b181980b9e6f24ca7dd357f5e3)
4 files changed