adb: overlayfs failed to remount /system

Regression introduced commit bb1efbd1

By not first checking if overlayfs mount on /system is in /proc/mounts
remount fails to discover the override and take the selected action.

Test: adb-remount-test.sh
Bug: 109821005
Change-Id: I1f0bae22caae1bca84fc1e33b508b54d98f4ed5c