Unmount public volume if FUSE mount fails

Sometimes, during early boot, a public volume may be created before
the user is unlocked and the mount may fail. This mount failure does
not revert the lower fs mounts (sdcardfs and vfat). Subsequent
mount attempts will then fail because we'd attempt to mount vfat on
already mounted /mnt/media_rw/<volname>

Bug: 158489548
Test: Resilient to an artificial sleep in
StorageManagerService#completeUnlockUser to
delay user unlock longer than public volume mount

Change-Id: I9a1574596434a2eb6b2553c0c9220c2118c7e4fd