Set foreground priority for shutdown receivers.

The broadcast of ACTION_SHUTDOWN has a timeout of 10 seconds. To
reduce the risk of hitting the timeout and speeding up the
shutdown sequence in ShutdownThread we set the priority of the
BroadcastReceivers to FLAG_RECEIVER_FOREGROUND.

We have seen that this change reduces the shutdown time when
the system is under heavy load.

Change-Id: I22cbf6af8cf6fc4bdefaa1c3da8a7eed7e7b7674