Enable referencing of aapt-generated proguard flags

Some targets may implement classes declared elsewhere, e.g., system
server implements services defined in framework-res.apk's manifest.
Allow depending on the aapt-generated proguard flags for a given
target to support this.

Bug: 272495195
Test: m + reference ":framework-res{.aapt.proguardOptionsFile}"
Change-Id: I2a16632ed6e5a9bddbe326bdb7ab0dd79b45e587
diff --git a/java/app.go b/java/app.go
index da9c6f3..29a2287 100755
--- a/java/app.go
+++ b/java/app.go
@@ -976,6 +976,10 @@
 // For OutputFileProducer interface
 func (a *AndroidApp) OutputFiles(tag string) (android.Paths, error) {
 	switch tag {
+	// In some instances, it can be useful to reference the aapt-generated flags from another
+	// target, e.g., system server implements services declared in the framework-res manifest.
+	case ".aapt.proguardOptionsFile":
+		return []android.Path{a.proguardOptionsFile}, nil
 	case ".aapt.srcjar":
 		return []android.Path{a.aaptSrcJar}, nil
 	case ".export-package.apk":