Hold partial wakelock during shutdown to avoid entering sleep

The ShutdownThread can get suspended while in progress if the
device enters sleep by the user pressing the power-key, or if it is
started (in sleep) from the BatteryService upon a dead battery
notification. If the device is woken up before the battery is
drained, the ShutdownThread will resume and finally complete
the shutdown, but if not the phone will stay in sleep until the
battery level is so low that the power is ruthlessly cut.

Change-Id: If64429fd0c98a9136141942be6c337b5c79cf4f1