Write ro.build.ab_update to /vendor/build.prop.

It's a vendor-specific property, which was historically included into
/system/build.prop prior to this change.

Whether a target uses A/B OTA shouldn't affect anything on the system
image, including the `ro.build_ab_update` property. Moving it to vendor
partition will also make it consistent with other A/B specific configs,
such as the `slotselect` flag in device fstab
(/vendor/etc/fstab.$(PRODUCT_PLATFORM)).

Bug: 130516531
Test: Build and flash crosshatch-userdebug. Check /system/build.prop,
      /vendor/build.prop and the runtime property.
Change-Id: I927625fbcc02c4a875a1f39850b51576f5ff6c66