Update droidstubs build target

This allows to use Metalava to generate metadata files useful for
Android Studio as part of a droidstubs target.
Once those files have been created in a new metadata folder, they are
zipped to make it easier to transfer them into the
out/target/common/obj/PACKAGING folder where they can then be picked up
by the SDK build to be included there.

Bug: 142480924
Test: m sdk
Change-Id: I4be1c9e78369c65ee9cd94706c6d20ab0df6b797
diff --git a/java/androidmk.go b/java/androidmk.go
index 5067e2f..58b9b80 100644
--- a/java/androidmk.go
+++ b/java/androidmk.go
@@ -534,6 +534,9 @@
 				if dstubs.jdiffDocZip != nil {
 					entries.SetPath("LOCAL_DROIDDOC_JDIFF_DOC_ZIP", dstubs.jdiffDocZip)
 				}
+				if dstubs.metadataZip != nil {
+					entries.SetPath("LOCAL_DROIDDOC_METADATA_ZIP", dstubs.metadataZip)
+				}
 				apiFilePrefix := "INTERNAL_PLATFORM_"
 				if String(dstubs.properties.Api_tag_name) != "" {
 					apiFilePrefix += String(dstubs.properties.Api_tag_name) + "_"