fs_mgr: overlayfs: system as root can not detect shared blocks (taimen)

init first stage manufactures a / mount pointing to /dev/root and
we can not use that for checking the filesystem.  Result is /system
overlay is mounted in second stage.

This adjustment to special case /dev/root is safe for overlayfs
handling because we will also gate on content in
/mnt/scratch/overlay/system, which will not be set during runtime
setup because check will have the proper block device at that time.

Test: adb-remount-test.sh
Bug: 123079041
Change-Id: I9c6674163955155f26ad2b0c9b78e9ff6507a514