Allow overriding SOONG_CONFIG_art_module_source_build to false.

Useful for local testing etc.

Test: env SOONG_CONFIG_art_module_source_build=false \
          MODULE_BUILD_FROM_SOURCE=true \
        build/soong/soong_ui.bash \
          --dumpvar-mode SOONG_CONFIG_art_module_source_build
  return false
Bug: 172480615
Change-Id: I50187408a01e0c6f5ecea12fe3a423e1c5d93612
diff --git a/core/android_soong_config_vars.mk b/core/android_soong_config_vars.mk
index 6fb14cd..6a0deae 100644
--- a/core/android_soong_config_vars.mk
+++ b/core/android_soong_config_vars.mk
@@ -44,7 +44,9 @@
   $(call add_soong_config_namespace,art_module)
   SOONG_CONFIG_art_module += source_build
 endif
-ifneq (,$(findstring .android.art,$(TARGET_BUILD_APPS)))
+ifneq (,$(SOONG_CONFIG_art_module_source_build))
+  # Keep an explicit setting.
+else ifneq (,$(findstring .android.art,$(TARGET_BUILD_APPS)))
   # Build ART modules from source if they are listed in TARGET_BUILD_APPS.
   SOONG_CONFIG_art_module_source_build := true
 else ifeq (,$(filter-out modules_% mainline_modules_%,$(TARGET_PRODUCT)))