Add ro.product.ab_ota_partitions as a build prop

The prop list the name of the a/b partitions that are supposed to
update via an OTA. The list varies by product, and update engine
needs to know these partitions to install the GKI update correctly.

Test: build and check the property
Change-Id: I5258955a5c3303bdc61b97fc92f5dfa1905f7c37