Export Make config vars to Soong to support Droiddoc

export Android.mk config vars to Soong, and export stubs.jar to Make
after compiling Droiddoc from Soong.

Bug: b/70351683
Test: Convert libcore docs Android.mk to Soong manually.

Change-Id: I749b4b3e1e50abce8a757130987c18d7135a3859
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index c3694ab2..506ed2d 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -239,6 +239,8 @@
 LOCAL_SOONG_PROGUARD_DICT :=
 LOCAL_SOONG_RESOURCE_EXPORT_PACKAGE :=
 LOCAL_SOONG_RRO_DIRS :=
+LOCAL_DROIDDOC_STUBS_JAR :=
+LOCAL_DROIDDOC_DOC_ZIP :=
 # '',true
 LOCAL_SOURCE_FILES_ALL_GENERATED:=
 LOCAL_SRC_FILES:=