Begin refactor of MediaFocusControl

Extract class that handles each entry in the remote control stack
  and move it to another file, MediaController.java.
Rename RemoteControlStackEntry to MediaController as each instance
  will not just encapsulate information about the corresponding
  (if any) RemoteControlClient.
This is just a CL for the renaming and extraction into a new file
  of existing code. Obvious required changes are labelled "FIXME".

Change-Id: Ifbdac1d70e4d279ab175eef03e9d792d44873c51