init: Bind mount /mnt/installer early for scoped storage.

Scoped storage has some unique requirements that are hard to implement
with the two mount namespaces, because the daemon that does the mounting
(vold) lives in a different namespace than the processes using those
mounts.

In particular, /mnt/installer is a special bind mount that should
receive mount events under /mnt/user, but at the same time only
only propagate mount events under /mnt/installer to /mnt/installer in
the other namespace. More details in the code.

Bug: 134706060
Test: /mnt/installer shows up and is setup correctly.
Change-Id: I6dab5ace5a345d9d684a9f1ae94c833fc294d49e