cas: fix UAF in descrambler -- DO NOT MERGE

Change the plugin holder in both CasImpl and DescramblerImpl
to shared_ptr, and use atomic store/load for read/write.

bug: 73172817
Test:CTS MediaCasTest, VTS VtsHalCasV1_0Target, poc in bug
Change-Id: I6b040680c28c38cef2fef3042f570dc64e86bb77
(cherry picked from commit addcb3aa)