Add new shutdown observer for MountService.

Use new observer before rebooting and shutting down.
Add some unit tests for unmount and shutdown code paths
Fix registering/unregistering part in MountService
Use ShutdownThread in PowerManager.reboot()
Add reboot support to ShutdownThread.
Remove MountService code from PowerManagerService.java and Power.java.
Clean shutdown/reboot is handled exclusively by ShutdownThread now.

Change-Id: Iefb157451d3d9c426cb431707b870a873c09123d