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Any ASEC or OBB files were unmounted when USB storage was set to UMS
mode. This changes it so only ASEC files on external storage and OBB
files mounted from external storage are unmounted.

(Cherry-pick of 93ecb38d)

Bug: 6948035
Change-Id: Ib60727bd360caa32173797ff5b4e1e21fcf20054