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
de23f56360