Add keys for networkstack

Resolves error
FAILED: ninja: 'user-keys/networkstack.x509.pem', needed by 'out/soong/.intermediates/packages/modules/CaptivePortalLogin/CaptivePortalLogin/android_common/CaptivePortalLogin.apk', missing and no known rule to make it

See:
https://android-review.googlesource.com/q/I7257a472d702e82acdadffc4e0535c63a8bd591d