public Intent prepare() {
        return VpnService.prepare(mApp);
    }
Prepare to establish a VPN connection. This method returns {@code null} if the VPN
application is already prepared or if the user has previously consented to the VPN
application. Otherwise, it returns an {@link Intent} to a system activity. The application
should launch the activity using {@link Activity#startActivityForResult} to get itself
prepared.

@return The intent to call using {@link Activity#startActivityForResult}.
netbare-core/src/main/java/com/github/megatronking/netbare/NetBare.java