Support bind mounting volumes into other volume's mountpoint.

With the way the FUSE mount point are currently setup for emulated
volumes, there can be multiple paths that serve the same files on the
lower filesystem; eg

* /mnt/user/0/emulated/0/Android
* /mnt/user/10/emulated/0/Android

both refer to the same file on the lower filesystem:
* /data/media/0/Android

this is normally not a problem, because cross-user file access is not
allowed, and so the FUSE daemon won't serve files for other users.

With clone profiles this is no longer true however, as their volumes
are accessible by each other.

So, it can happen that an app running in clone profile 10 accesses
"/mnt/user/10/emulated/0/Android", which would be served by the FUSE
daemon for the user 10 filesystem.

At the same time, an app running in the owner profile 0 accesses
"mnt/user/0/emulated/0/Android", which would be served by the FUSE
daemon for the user 0 filesystem.

This can cause page cache inconsistencies, because multiple FUSE daemons
can be running on top of the same entries in the lower filesystem.

To prevent this, use bind mounts to make sure that cross-profile
accesses actually end up in the FUSE daemon to which the volume
belongs: "/mnt/user/10/emulated/0" is bind-mounted to
"/mnt/user/0/emulated/0", and vice-versa.

Bug: 228271997
Test: manual
Change-Id: Iefcbc813670628b329a1a5d408b6126b84991e09