Address multiple RemoteControlDisplay competing for registration

The RemoteControlClient / Display feature only supports one
 display. If multiple displays are registered, this CL implements
 the following policy:
 - cannot unregister a display that is not the current one,
 - registering a display when another is already registered
    causes the old one to be unregistered.
This fixes a death handler leak where the previous display was
 simply overwritten, without unlinking to its death.

Change-Id: I63f8a38093796e0960761936d7fc58d47b7589b3