use ConnectivityManager to improve swap reliability

ConnectivityManager had an overhaul in android-21 and provides some features that could be useful for making WiFi swapping more reliable:

• https://developer.android.com/reference/android/net/ConnectivityManager.html
• https://developer.android.com/reference/android/net/Network.html#openConnection(java.net.URL,%20java.net.Proxy)