Let update_verifier work on non-AB update devices

Make update_verifier check if it runs on A/B update devices at the
beginning, and quit immediately if it doesn't, instead of re-boot.

Bug: 70541023
Test: On aosp/master:
    $ lunch aosp_x86_64-userdebug; m -j # boot to home screen
    # On goog/master:
    $ lunch aosp_walleye-userdebug; m -j # boot to home screen

Change-Id: Ib71a3a3b272cfa5dd0b479eaa067eedaec8fde7d