fs_mgr: overlay: do not allow sub-mounts to be overridden

Prevent sub-mounts from being candidates for overlayfs overrides.
We move them temporarily to hold the references, then move them back
after overlayfs mount.

There is a race condition that can not be solved where the content
of the sub-mount is temporarily unavailable.  This operation occurs
in first stage init (no problem) and during the first adb remount or
adb disable-verity that causes the backing storage to be setup.
Workaround will be to time the adb remount/disable-verity so that the
device-specific problems do not surface.

Test: adb-remount-test.sh and manual device configs
Bug: 130131892
Bug: 135647457
Merged-In: I8782d09b7b389d899aed07b3e6c528280af2d6c8
Change-Id: I8782d09b7b389d899aed07b3e6c528280af2d6c8