Sign networkstack with its own certificate

The NetworkStack module is comprised of 2 updatable APKs
(NetworkStack.apk and CaptivePortalLogin.apk), and a stub APK to define
the signature permissions for the APKs (NetworkStackPermissionStub.apk).

Test: flashed, WiFi works, captive portal login works
Bug: 124033493
Change-Id: I772d1d63363a3eb6f817c21906498bd2385ab02b
diff --git a/packages/NetworkStack/Android.bp b/packages/NetworkStack/Android.bp
index 855d5dd..b700bf3 100644
--- a/packages/NetworkStack/Android.bp
+++ b/packages/NetworkStack/Android.bp
@@ -35,7 +35,7 @@
 android_app {
     name: "NetworkStack",
     sdk_version: "system_current",
-    certificate: "platform",
+    certificate: "networkstack",
     privileged: true,
     static_libs: [
         "NetworkStackLib"