Sign demo app with the platform key

Because we'll now allow virtualizationservice permission only to
platform_apps.

Test: run demo app
Change-Id: I1d58403f363c777565f20ae81da447c2d6ce94b6
diff --git a/demo/Android.bp b/demo/Android.bp
index 77049de..37fc676 100644
--- a/demo/Android.bp
+++ b/demo/Android.bp
@@ -18,4 +18,5 @@
     platform_apis: true,
     use_embedded_native_libs: true,
     v4_signature: true,
+    certificate: "platform",
 }