lineage: use AB_OTA_PARTITIONS flag for including backuptool_ab

* Newer pixel devices get AB_OTA_UPDATER enabled from
  build/make/target/board/BoardConfigMainlineCommon.mk which
  is not included in time for this check.

Change-Id: I1679709b7178398ffaa62ad14a6d042ff377fd5a
1 file changed