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/soong_config.mk b/core/soong_config.mk
index a084f79..8b2dfd1 100644
--- a/core/soong_config.mk
+++ b/core/soong_config.mk
@@ -42,6 +42,9 @@
 
 $(call add_json_str,  Make_suffix, -$(TARGET_PRODUCT))
 
+$(call add_json_str,  BuildId,                           $(BUILD_ID))
+$(call add_json_str,  BuildNumberFromFile,               $$$(BUILD_NUMBER_FROM_FILE))
+
 $(call add_json_val,  Platform_sdk_version,              $(PLATFORM_SDK_VERSION))
 $(call add_json_csv,  Platform_version_active_codenames, $(PLATFORM_VERSION_ALL_CODENAMES))
 $(call add_json_csv,  Platform_version_future_codenames, $(PLATFORM_VERSION_FUTURE_CODENAMES))