Fix sandman crash

It was possible for the system to crash when trying to handle sandman
because the DisplayGroupPowerStateMapper was being accessed without
holding the Power Lock.

This change ensures that DisplayGroupPowerStateMapper is only accessed
while holding the lock which prevents this crash from happening.

Bug: 185419869
Change-Id: I96ca47335430023b6e2ac4b6f6e499fa3dc5a53b