Progress towards dynamic storage support.

Storage devices are no longer hard-coded, and instead bubble up from
whatever Disk and VolumeBase that vold uncovered, turning into
sibling Java objects in MountService.  We now treat vold events as
the source-of-truth for state, and synchronize our state by asking
vold to "reset" whenever we reconnect.

We've now moved to a model where all storage devices are mounted in
the root mount namespace (user boundaries protected with GIDs), so
we no longer need app-to-vold path translation.  This also means that
zygote only needs to bind mount the user-specific /mnt/user/n/ path
onto /storage/self/ to make legacy paths like /sdcard work.  This
grealy simplifies a lot of system code.

Many parts of the platform depend on a primary storage device always
being present, so we hack together a stub StorageVolume when vold
doesn't have a volume ready yet.

StorageVolume isn't really a volume anymore; it's the user-specific
view onto a volume, so MountService now filters and builds them
based on the calling user.  StorageVolume is now immutable, making
it easier to reason about.

Environment now builds all of its paths dynamically based on active
volumes.  Adds utility methods to turn int types and flags into
user-readable strings for debugging purposes.

Remove UMS sharing support for now, since no current devices support
it; MTP is the recommended solution going forward because it offers
better multi-user support.

Simplify unmount logic, since vold will now gladly trigger EJECTING
broadcast and kill stubborn processes.

Bug: 19993667
Change-Id: I9842280e61974c91bae15d764e386969aedcd338