Export PRODUCT_SOONG_NAMESPACES to Soong

Bug: 65683273
Test: put 'PRODUCT_SOONG_NAMESPACES := jeff-test/dir1' \
      in a .mk file and observe its value copied into \
      out/soong/soong.variables

Change-Id: I2b61caa46f40f35ccecf64da88918b73180e02e8
diff --git a/core/soong_config.mk b/core/soong_config.mk
index a90e5af..6f5ca4e 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -121,6 +121,8 @@
 
 $(call add_json_str,  DistDir,                           $(if $(dist_goal), $(DIST_DIR)))
 
+$(call add_json_list, NamespacesToExport,                $(PRODUCT_SOONG_NAMESPACES))
+
 _contents := $(subst $(comma)$(newline)__SV_END,$(newline)}$(newline),$(_contents)__SV_END)
 
 $(file >$(SOONG_VARIABLES).tmp,$(_contents))