Progress towards dynamic storage support.

To support external storage devices that are dynamically added and
removed at runtime, we're changing /mnt and /storage to be tmpfs that
are managed by vold.

To support primary storage being inserted/ejected at runtime in a
multi-user environment, we can no longer bind-mount each user into
place.  Instead, we have a new /storage/self/primary symlink which
is resolved through /mnt/user/n/primary, and which vold updates at
runtime.

Fix small mode bugs in FUSE daemon so it can be safely mounted
visible to all users on device.

Bug: 19993667
Change-Id: I0ebf4d10aba03d73d9a6fa37d4d43766be8a173b