ignore RemoteException from pm if shutdown is already on-going

- In new shutdown flow, adb is guaranteed to be alive while system server
  is alive.
- power manager can be killed before system server is killed.
- This leads into RemoteException for shutdown request to power manager.
- If shutdown is already on-going (=sys.powerctl set), the RemoteException
  should be ignored.

bug: 37096155
Test: adb shell svc power reboot|shutdown and check output to adb
Change-Id: Ia90a900d55123bd7c1bf57c90c49d35332ebfdf5