Part of media security hardening

This is an intermediate step toward moving
mediadrm to a new service separate from mediaserver.
This first step allows mediadrmservice to run based
on the system property media.mediadrmservice.enable
so it can be selectively enabled on devices that
support using native_handles for secure buffers.

bug: 22990512
Change-Id: I2208c1e87a6bd8d5bfaed06b1fdcb0509c11cff2