fs_mgr_overlayfs: Don't preemptively change mount propagation type

If the overlaid mount have no submount and is not submount of other
mount, then we don't need to change its propagation type.
This makes the remount process less disruptive as we create less
fragmented mount namespaces.
The general rule is still to always reboot after initial remount setup
to get the most predictable result.

Bug: 342967841
Test: adb_remount test
Change-Id: I19ca62018149f1fa81186824a9d7f1c32d9d6008