fs_mgr: overlayfs: test: skip apex mounts

Strip out /dev/block/loop* devices to filter out apex mounts.  Deal
with scale of skip_administrative_mounts function to make for easier
maintenance.

Test: adb-remount-test.sh
Bug: 109821005
Bug: 112455435
Change-Id: Ic7382a613db8f3ac7cdc10721194042378d136b9