Fix issue #2541849: System crash when an app with active notifications...

...in status bar is installed on sdcard and device is powered down.

Don't do the GC until everyone has had a chance to take care of external
storage going away.

This is more correct, but doesn't completely fix the problem that
appears when shutting down.

Change-Id: I24b19337ba6262cfd8d010645dbf67e76ea03058