Enable AB_OTA_UPDATER for mainline

The file $(TARGET_OUT_ETC)/update_engine/update-payload-key.pub.pem
is installed conditionally based on this variable, and was found
to differ between mainline_system and the intended device targets.

Also move the variable from gsi_common.mk as GSI inherits the mainline
boardconfig. The variable is not a PRODUCT flag (and not inherited as
such), so fits better in the BoardConfig I think.

Bug: 80410283
Test: presubmit
Change-Id: Ida6fa923fd01b062ff3b93e7737e1e6edf7e4fa1