Public volumes belong to a single user.

When a public (vfat) device is inserted, it's strongly associated
with the current foreground user, and no other users should be able
to access it, since otherwise that would be a cross-user data leak.

To use the device under a different user, switch users and then
eject/remount the device.

Test: verified user isolation of USB drive
Bug: 32523490
Change-Id: I590c791996f1fea8d78f625dc942d149f1f41614