Gracefully handle null volumes after forgetting.

If a user tells a device to forget a storage volume and then
goes back to the PrivateVolumeForget activity from their Recents, it
attempts to re-initialize itself with a volume that it forgot.

And this makes Settings crash.

By gracefully killing the activity when we try to open
PrivateVolumeForget for a forgotten volume, we avoid this crash.

Change-Id: Ib4e881c10f0c872ce6b268b16a573960230ef99b
Fixes: 34856304
Test: Settings unit test