Fix NPE during startup when battery is low

During startup the mConfiguration in the ActivityThread for the
system_server process is null until the CONFIGURATION_CHANGED
message is received. This message is sent by the
WindowManagerService.PolicyThread (and later another one is sent
by the SystemServer.ServerThread). If the PolicyThread happens to be
scheduled a bit later for some reason the LAUNCH_ACTIVITY message
for the ShutdownActivity will be received first, causing a NPE in
performLaunchActivity when executing new Configuration(mConfiguration);

Change-Id: I004b02038ba0e55ded399b3b2fe5b74ad0a76c78