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
18e7bce523