Remove MediaDrm stack from mediaserver

Part of media security hardening

MediaDrm was temporarily allowed to
run in either mediaserver or mediadrmserver
while all nexus devices were implementing
support for the native_handle representation
of secure buffers.  Now that all nexus
devices support native_handle, the MediaDrm
stack can be removed from mediaserver.

b/22990512

Change-Id: I96a5222f94cd1580ac411de5b48beb44b99cc8d7