Change the condition for building super_empty.img.

This CL changes the condition for building super_empty.img from
PRODUCT_BUILD_SUPER_PARTITION to PRODUCT_USE_DYNAMIC_PARTITIONS, as a
follow-up to the change in [1].

With the CL in [1], it skips building super.img and super_empty.img both
when turning off PRODUCT_BUILD_SUPER_PARTITION. However, the latter
should be mandatory whenever dynamic partitions is enabled. Because
fastboot relies on this file to properly flash dynamic partitions. Plus,
the cost for building super_empty.img is much lower than the one for
super.img.

As part of the change, it'll write group info into target_files when
building with PRODUCT_BUILD_SUPER_PARTITION == false. It's the work for
target_files merging script to determine the values to be picked up. The
current logic in merge_target_files.py always uses the one from vendor
target_files. This CL adds a testcase to ensure the behavior.

[1] https://android-review.googlesource.com/c/platform/build/+/928756

Bug: 135752763
Test: `m dist` with a target that sets PRODUCT_BUILD_SUPER_PARTITION to
      false. Check the built artifacts contain super_empty.img. Verify
      that the build can be flashed properly.
Change-Id: I277f087eab45663a6c3b33333d16e9e576c1c25c
diff --git a/core/Makefile b/core/Makefile
index e921dfa..838a96c 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -4536,6 +4536,19 @@
 
 endif # TARGET_BUILD_APPS
 
+
+ifeq (true,$(PRODUCT_USE_DYNAMIC_PARTITIONS))
+
+# Dump variables used by build_super_image.py (for building super.img and super_empty.img).
+# $(1): output file
+define dump-super-image-info
+  $(call dump-dynamic-partitions-info,$(1))
+  $(if $(filter true,$(AB_OTA_UPDATER)), \
+    echo "ab_update=true" >> $(1))
+endef
+
+endif # PRODUCT_USE_DYNAMIC_PARTITIONS
+
 # -----------------------------------------------------------------
 # super partition image (dist)
 
@@ -4544,13 +4557,6 @@
 # BOARD_SUPER_PARTITION_SIZE must be defined to build super image.
 ifneq ($(BOARD_SUPER_PARTITION_SIZE),)
 
-# Dump variables used by build_super_image.py.
-define dump-super-image-info
-  $(call dump-dynamic-partitions-info,$(1))
-  $(if $(filter true,$(AB_OTA_UPDATER)), \
-    echo "ab_update=true" >> $(1))
-endef
-
 ifneq (true,$(PRODUCT_RETROFIT_DYNAMIC_PARTITIONS))
 
 # For real devices and for dist builds, build super image from target files to an intermediate directory.
@@ -4631,7 +4637,7 @@
 # -----------------------------------------------------------------
 # super empty image
 
-ifeq (true,$(PRODUCT_BUILD_SUPER_PARTITION))
+ifeq (true,$(PRODUCT_USE_DYNAMIC_PARTITIONS))
 ifneq ($(BOARD_SUPER_PARTITION_SIZE),)
 
 INSTALLED_SUPERIMAGE_EMPTY_TARGET := $(PRODUCT_OUT)/super_empty.img
@@ -4647,7 +4653,7 @@
 $(call dist-for-goals,dist_files,$(INSTALLED_SUPERIMAGE_EMPTY_TARGET))
 
 endif # BOARD_SUPER_PARTITION_SIZE != ""
-endif # PRODUCT_BUILD_SUPER_PARTITION == "true"
+endif # PRODUCT_USE_DYNAMIC_PARTITIONS == "true"
 
 
 # -----------------------------------------------------------------