adb-remount-test: Simplify disable-verity setup

Don't try to parse stdout of disable-verity, just pass -R to ask for
auto reboot.
This eliminates the complex logic of "disable-verity && check &&
reboot && disable-verity again ...", and increase robustness.

`${overlayfs_supported} && ${overlayfs_needed}` can be simplified to
just `${overlayfs_needed}` because `${overlayfs_needed}` implies
`${overlayfs_supported}`.

Move the curious recurring "overlay takeover unexpected" check to the
"Checking current overlayfs status" section so we don't need to repeat
it so many times.

Bug: 243116800
Bug: 241688845
Test: adb-remount-test
Change-Id: I96ec44e2b9d172c06c3b4850e061e7b6bb46833c