Debugger: Make registration of services explicit

Debugger: Make registration of services explicit

Services now have to call registerService() themselves in the constructor.
This fixes a race condition where the empty implementation of messageReceived()
was called instead of the one in the subclass because the object wasn't fully
constructed yet.

Change-Id: I590ec8b76e906bdb6b5cdcb18680938edde283ee
Reviewed-by: Aurindam Jana <aurindam.jana@nokia.com>