Move all targets away from non-AB

This sets AB_OTA_UPDATER to true by default. OEMs can still
override this variable in their board config if their device
is still on non-AB.

Since some targets(e.g. art target) do not need updates,
we permit empty AB_OTA_PARTITIONS if AB_OTA_UPDATER is true.

This reverts commit 2a3faafd

Test: th
Bug: 324360816
Change-Id: Ia9e30983ddc7981659d042a177e9edc8abac6a47