fs_mgr: overlayfs: pre-emptive filesystem setup

If we just disabled verity, take the step to setup the backing
storage for overlayfs.

Test: adb-remount-test.sh
Bug: 130131892
Change-Id: Ic56569eaf363e4417a0ce5df11ee5480a07b8b03
Merged-In: Ic56569eaf363e4417a0ce5df11ee5480a07b8b03