fs_mgr_overlayfs: Fix submount propagation type after remount

When juggling submounts, the temp dir holding the references to
submounts should be MS_PRIVATE. Otherwise the submount propagation type
would be changed according to section 5d,
https://www.kernel.org/doc/Documentation/filesystems/sharedsubtree.txt

For example, a root mount tree of
  <mountpoint> <shared/private>
  /dev         shared
  /product     shared
  /product/app private

When executing "remount /product", "/product/app" would be moved to a
temp dir under "/dev", which is MS_SHARED. This would also change the
state of the mount to MS_SHARED.
  /dev         shared
  /product     shared
  /dev/temp    shared  (moved from /product/app)

Thus after "/product" overlay is mounted, and "/dev/temp" is moved back,
"/product/app" would be unintentionally changed to MS_SHARED.
  /dev         shared
  /product     private
  /product     shared  (overlayfs override)
  /product/app shared  (moved from /dev/temp)


This change fixes this issue by setting up a standalone mount tree under
/dev to hold the temporary moved submounts.
This way we don't need to modify the state of "/dev" whatsoever, and can
guarantee the propagation type of mounts moved in and out of the temp
dir are not changed.

Bug: 306124139
Test: adb-remount-test
Test: remount a parent mount and then verify the mount flag of submount
Change-Id: I8174257cff6b93dd95303c6ab49b42f90b02df0c