Adding Build time flag to specify Cow Version

Adding in build time flag for specifying cow version, in preparation to
add new cow version in libsnapshot

Test: m -> cat misc-info.txt
Bug: 279954153
Change-Id: I148d4e7cdd669496c91213889787172ffd83a5ab

core/Makefile

@@ -5763,6 +5763,8 @@ define dump-dynamic-partitions-info
     echo "virtual_ab_compression_method=$(PRODUCT_VIRTUAL_AB_COMPRESSION_METHOD)" >> $(1))
   $(if $(filter true,$(PRODUCT_VIRTUAL_AB_OTA_RETROFIT)), \
     echo "virtual_ab_retrofit=true" >> $(1))
+  $(if $(PRODUCT_VIRTUAL_AB_COW_VERSION), \
+    echo "virtual_ab_cow_version=$(PRODUCT_VIRTUAL_AB_COW_VERSION)" >> $(1))
 endef
 
 # By conditionally including the dependency of the target files package on the

core/product.mk

@@ -401,6 +401,10 @@ _product_list_vars += PRODUCT_VENDOR_LINKER_CONFIG_FRAGMENTS
 #   supports it
 _product_single_value_vars += PRODUCT_ENABLE_UFFD_GC
 
+# Specifies COW version to be used by update_engine and libsnapshot. If this value is not
+# specified we default to COW version 2 in update_engine for backwards compatibility
+_product_single_value_vars += PRODUCT_VIRTUAL_AB_COW_VERSION
+
 _product_list_vars += PRODUCT_AFDO_PROFILES
 
 .KATI_READONLY := _product_single_value_vars _product_list_vars

target/product/virtual_ab_ota/android_t_baseline.mk

@@ -20,3 +20,5 @@
 #
 # All U+ launching devices should instead use vabc_features.mk.
 $(call inherit-product, $(SRC_TARGET_DIR)/product/virtual_ab_ota/vabc_features.mk)
+
+PRODUCT_VIRTUAL_AB_COW_VERSION := 2