fs_mgr: overlayfs: dig harder for /dev/root equivalent

Check for A/B /dev/block/by-name/system<slot> and also non-A/B
/dev/block/by-name/system to discover what /dev/root can be when
evaluating candidates for using overlayfs.

This is to handle a misconfigured (or legacy) system-as-root device.
It is recommended that the default fstab specifically mentions the
root mount's device node to prevent going down this path.

Test: adb-remount-test.sh
Bug: 138407617
Change-Id: I3853d203b9376d0f848cb490150ff00cc4ed3d5f