fs_mgr: adb-remount-test: non-verity recovery

For devices that do not have verity, or start the test with
verity disabled, but are using overlayfs, use a 'secret' surgical
technique to revert the device back to original state.

adb enable-verity will carry risk in that if the device did not
arrive with verity enabled, we can not guarantee it will boot
if we turn on verity.

Test: atest adb-remount-sh
Bug: 138649540
Change-Id: If6d55c57fb9daca9305c42a4df2c18c7f8c5eefa