fs_mgr: skip fs_mgr_overlayfs_required_devices w/o overlayfs in kernel

Check if overlayfs is supplied in the kernel before proceeding to
determining if there is a disabled verity and an overlayfs filesystem
to deploy.

Test: adb-remount-test.sh
Bug: 119929969
Change-Id: I28116f0aa6959040bb9f38f46c058a221591f735