Fix MediaDrm security level APIs

During development of the vendor/widevine HAL module, the newly added
MediaDrm.setSecurityLevel method was found to be difficult to
implement efficiently since the security level is needed at the time
a session is opened. This CL moves the security level parameter to
openSession to resolve this problem.

bug:72831697
bug:64001680

test: GTS media tests
Change-Id: I74b3d1013405805c18b9794bbf7c46348e0ccda9