[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
am: 8d7de50654

Change-Id: I57e923fe69cd4da8e0884d5d1c8d66f5bb387efa