Use ART_MODULE_BUILD_FROM_SOURCE instead soong config variable

When product configuration is converted to Starlark, soong config
variables will be stored in a dictionary and won't be able to be
specified directly on the command line. Use a regular variable
that controls the soong config variable.

Bug: 220940864
Test: Presubmits
Change-Id: I5622d9e626e2cf84afe67cd2a214530e64cefc3e
diff --git a/Android.mk b/Android.mk
index 47d0a2b..dc5b0c8 100644
--- a/Android.mk
+++ b/Android.mk
@@ -100,7 +100,7 @@
 
 ########################################################################
 # Everything below is only available in ART source builds
-# (SOONG_CONFIG_art_module_source_build=true).
+# (ART_MODULE_BUILD_FROM_SOURCE=true).
 ########################################################################
 
 # TODO(b/172480617): Clean up the platform dependencies on everything above and
@@ -394,10 +394,10 @@
 
 # Reference the libraries and binaries in the appropriate APEX module, because
 # they don't have platform variants. However if
-# SOONG_CONFIG_art_module_source_build isn't true then the APEX modules are
-# disabled, so Soong won't apply the APEX mutators to them, and then they are
-# available with their plain names.
-ifeq (true,$(SOONG_CONFIG_art_module_source_build))
+# ART_MODULE_BUILD_FROM_SOURCE isn't true then the APEX
+# modules are disabled, so Soong won't apply the APEX mutators to them, and
+# then they are available with their plain names.
+ifeq (true,$(ART_MODULE_BUILD_FROM_SOURCE))
   art_module_lib = $(1).com.android.art
   art_module_debug_lib = $(1).com.android.art.debug
 else