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