Add docs.zip output tag to droiddoc

Let genrules access the docs.zip output using ":module{.docs.zip}".

Bug: 138396395
Test: m checkbuild
Change-Id: I71e22147a596227f21826d875926b8b2ad1c74c1

java/droiddoc.go

@@ -359,6 +359,8 @@ func (j *Javadoc) OutputFiles(tag string) (android.Paths, error) {
 	switch tag {
 	case "":
 		return android.Paths{j.stubsSrcJar}, nil
+	case ".docs.zip":
+		return android.Paths{j.docZip}, nil
 	default:
 		return nil, fmt.Errorf("unsupported module reference tag %q", tag)
 	}