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/product.mk b/core/product.mk
index 2276db2..c7aac00 100644
--- a/core/product.mk
+++ b/core/product.mk
@@ -381,6 +381,8 @@
 # If set, Java module in product partition cannot use hidden APIs.
 _product_single_value_vars += PRODUCT_ENFORCE_PRODUCT_PARTITION_INTERFACE
 
+_product_single_value_vars += PRODUCT_INSTALL_EXTRA_FLATTENED_APEXES
+
 .KATI_READONLY := _product_single_value_vars _product_list_vars
 _product_var_list :=$= $(_product_single_value_vars) $(_product_list_vars)