Export AB_OTA_PARTITION to soong migration vars

This will be used to generate ab_partitions.txt in soong's
target_files.zip

Bug: 388633394
Test: Built soong's target_files.zip locally
Change-Id: Iaf18f0afe8b8cde8eccb44e729670576792e8923
diff --git a/core/soong_config.mk b/core/soong_config.mk
index dcd654d..c90860f 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -485,6 +485,7 @@
   $(call add_json_str, ProductVirtualAbCompressionFactor, $(PRODUCT_VIRTUAL_AB_COMPRESSION_FACTOR))
   $(call add_json_str, ProductVirtualAbCowVersion, $(PRODUCT_VIRTUAL_AB_COW_VERSION))
   $(call add_json_bool, AbOtaUpdater, $(filter true,$(AB_OTA_UPDATER)))
+  $(call add_json_list, AbOtaPartitions, $(AB_OTA_PARTITIONS))
 
   # Avb (android verified boot) stuff
   $(call add_json_bool, BoardAvbEnable, $(filter true,$(BOARD_AVB_ENABLE)))