Free AssetManager weak refs before unmounting

AssetManagers are kept in WeakReference pools so there's a change
another thread in the system_server has opened a file on the external
card. Attempt to force a garbage collection to free these references
each time we're about to unmount a volume on the external storage.

Change-Id: I6e816e8c67b179887e488dc6758b43bcdd5914fb