[DO NOT MERGE] Enforce user separation on external storage am: cc1cecc9be am: 5360f2f4e8 am: 7beb5adcd0 am: c11f30ff5c am: f5b7af30f2
am: f88c3ff9ad  -s ours

Change-Id: I26038bc5e039d4f35766d52db2311138e95963d3