set-verity-state: Simplify code path around overlayfs_setup()

Original code:
  Calls |overlayfs_setup()| if |any_changed| is false
  |any_changed| is true implies |set_avb_verity_enabled_state()| is true
  which implies |overlayfs_setup()| is called by it.

All this means |overlayfs_setup()| is called exactly once regardless of
the value of |any_changed|, so simplify the logic to the above observation.

Bug: 241688845
Test: Presubmit
Change-Id: Ifcbae8db7fdda2f7576769cb4a213c305c670709