[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
am: 85e9579545

Change-Id: I5714bab0e73296133a8be8c041ca19b48cfcf40c