Fix secondary volume mounts on user switch

If a public volume is mounted when the foreground user is switched,
the old user is stopped but the public volume isn't unmounted so the
/mnt/user/<old user>/<volume> path still exists but is inaccessible,
even after the foreground user is switched back the old user.

Now, we remount the public volumes in such cases to fixup the mounts

Test: Manual
Bug: 183686263
Bug: 171460866
Change-Id: Ic1226229d377d7106bd78798a03adfc9b7f2efb6