fs_mgr: overlayfs: test fix problems with runtime apex bind mounts

bind mounts create administrivia in /proc/mounts that can not be
easily discerned to confirm expectations.  We squash the stutter,
and filter out overlay backing references for /system/<path>/<file>.

Test: adb-remount-test.sh
Bug: 122737045
Change-Id: I61342f5f76f54a1a8201a52efb8054acde100a6f