[DO NOT MERGE] Enforce user separation on external storage am: 5432cedb61 am: c3c4b15dee am: 7d869bd17f am: 0312164f4b  -s ours am: 45beb672fd am: aab2da8ce2
am: 7feb74def7

Change-Id: I8111601edd533dd83988b98ba6eef89a605bfb34