Make update_engine work with non-AB update devices

Disable update_engine service initially. Enable it only on A/B
update devices (by checking "ro.boot.slot_suffix" property).

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: I68bc1ddc9689a22a64dd1d96e884fdae30eb54f5