camera: Add AIDL interface for CameraServiceProxy.

- Adds an AIDL interface to allow the proxy camera service
  running in system server to accept RPCs from the camera
  service running in mediaserver.
- Request an update to the valid user set from the proxy
  camera service when mediaserver restarts to initialize
  properly + avoid DOS after a crash.

Bug: 21267484
Change-Id: Ib821582794ddd1e3574b5dc6c79f7cb197b57f10