Multi-user external storage support.

Remount rootfs as recursively shared, so that mount changes are
propagated into child namespaces.  Mount external storage for access
from adb.

Clean multi-user dependencies for use in Dalvik.  Also define
external storage paths.

Bug: 6925012
Change-Id: I375de581a63f4f36667894c56a34a9dd45361e8f