Explicitly set AB_OTA_UPDATER := true.

This was once removed in commit 2c8c16bd9763d32b9798c08ece365c105f49d5cc
to eliminate the duplicates between Pixel and mainline BoardConfigs.
This CL adds `AB_OTA_UPDATER := true` back since it's to be removed out
of the mainline BoardConfig.

Bug: 130516531
Test: TreeHugger
Change-Id: Ic8ec09c11014f554e6cfdedf067c41f60af527df
Merged-In: Ic8ec09c11014f554e6cfdedf067c41f60af527df
(cherry picked from commit 5c04c063493908bc7ea3aaa70cd8b91c0f02b422)
diff --git a/device.mk b/device.mk
index 524b52a..ffd4ce7 100644
--- a/device.mk
+++ b/device.mk
@@ -152,6 +152,8 @@
 PRODUCT_PROPERTY_OVERRIDES += \
     ro.cp_system_other_odex=1
 
+AB_OTA_UPDATER := true
+
 AB_OTA_PARTITIONS += \
     boot \
     system \