Fix bug 5357295 NPE on display when remote control client dies

When a IRemoteControlClient dies, that client is set to null
 in the stack of remote control entries (mRCStack). This is done
 by calling registerRemoteControlClient() with a null client.
The bug is that registerRemoteControlClient(), after storing
 the new client, uses it to let it know what the current
 remote control display is. When that display is non null, the
 client is sent the current display. So when a client died
 when there was a display, the client reference was accessed
 in the part of the method where we haven't yet checked whether
 it is null or not.
The fix consists in moving the setting of the display on the
 client (method plugRemoteControlDisplay) only after having
 checked that the client is non-null.

Change-Id: Ic74d6cba9e3a3a16e78cd80a1ae5901abfeb3905