Improve secondary volume mounts on user switch

Ic1226229d377d7106bd78798a03adfc9b7f2efb6 fixed the secondary volume
mounts on user switch but there some issues when switching to a user
that is not yet unlocked because the ExternalStorageService for the
user might not yet be ready.

Now, we fix this by checking if the user is unlocked and delaying the
remount till after unlock in such cases

Test: Manual
Bug: 183686263
Change-Id: I7c885b0c3e6cefe23ba405710c2a50ad957bff5c