Export the cert path for runtime_resource_overlay.

Fixes: 119811120
Test: app_test.go
Test: Converted and built IconPackFilledSystemUIOverlay
Change-Id: I3f7eae3127996773c5abf9f9bdb9d57a198ddaa3
diff --git a/java/app.go b/java/app.go
index a27996c..6e0ffeb 100755
--- a/java/app.go
+++ b/java/app.go
@@ -1243,6 +1243,8 @@
 
 	properties RuntimeResourceOverlayProperties
 
+	certificate Certificate
+
 	outputFile android.Path
 	installDir android.InstallPath
 }
@@ -1288,6 +1290,7 @@
 	certificates = processMainCert(r.ModuleBase, String(r.properties.Certificate), certificates, ctx)
 	signed := android.PathForModuleOut(ctx, "signed", r.Name()+".apk")
 	SignAppPackage(ctx, signed, r.aapt.exportPackage, certificates)
+	r.certificate = certificates[0]
 
 	r.outputFile = signed
 	r.installDir = android.PathForModuleInstall(ctx, "overlay", String(r.properties.Theme))