Fix MediaDrm security level APIs

Vendors implementing the newly added MediaDrm.setSecurityLevel
HAL found that it was difficult to implement properly. Also the
semantics are somewhat ambiguous from the Java platform API
level. This CL binds the security level assignment to openSession
which clarifies the API for apps as well as making it more
natural to implement in HAL modules.

bug:72831697
bug:64001680

test: VtsHalDrmV1_1TargetTest, GTS media tests
Change-Id: Iaa07727be86ec2bc92be907d5a48c92136dc2014