Add a tagged output for app's exportPackage

This allows the android stubs to depend on framework-res' exportPackage
without special-casing inside the build system.

Bug: 161214753
Test: depending on this output in a followup CL
Merged-In: I8c5d17540d2624974983d73e56ba17898505dba8
Change-Id: I8c5d17540d2624974983d73e56ba17898505dba8
diff --git a/java/app.go b/java/app.go
index e75d874..2ec27f3 100755
--- a/java/app.go
+++ b/java/app.go
@@ -969,6 +969,8 @@
 	switch tag {
 	case ".aapt.srcjar":
 		return []android.Path{a.aaptSrcJar}, nil
+	case ".export-package.apk":
+		return []android.Path{a.exportPackage}, nil
 	}
 	return a.Library.OutputFiles(tag)
 }