Add PRODUCT_INSTALL_EXTRA_FLATTENED_APEXES

For GSI targets, this variable is set true, and Soong will install both
"flattened" and "unflattened" APEXes together in /system_ext and /system
respectively.

Bug: 137802149
Test: lunch aosp_arm64-userdebug && m
  resulting apex images under /system/apex
  and flattened apexes under /system/system_ext/apex

Change-Id: Ib72d1b9dbd59727942da39323ee7e60ac6e14882
diff --git a/core/soong_config.mk b/core/soong_config.mk
index 7ce59f1..85e7bbc 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -199,6 +199,8 @@
 
 $(call add_json_bool, EnforceProductPartitionInterface,  $(PRODUCT_ENFORCE_PRODUCT_PARTITION_INTERFACE))
 
+$(call add_json_bool, InstallExtraFlattenedApexes, $(PRODUCT_INSTALL_EXTRA_FLATTENED_APEXES))
+
 $(call json_end)
 
 $(file >$(SOONG_VARIABLES).tmp,$(json_contents))