MediaCAS: adding media.cas to service

Also allow media.extractor to use media.cas for descrambling.

bug: 22804304

Change-Id: Id283b31badecb11011211a776ba9ff5167a9019d
diff --git a/private/service_contexts b/private/service_contexts
index 6f46793..5200b8d 100644
--- a/private/service_contexts
+++ b/private/service_contexts
@@ -87,6 +87,7 @@
 media.radio                               u:object_r:audioserver_service:s0
 media.sound_trigger_hw                    u:object_r:audioserver_service:s0
 media.drm                                 u:object_r:mediadrmserver_service:s0
+media.cas                                 u:object_r:mediacasserver_service:s0
 media_projection                          u:object_r:media_projection_service:s0
 media_resource_monitor                    u:object_r:media_session_service:s0
 media_router                              u:object_r:media_router_service:s0