More fine-grained checking of the internet connection.
This issues helps to fix #110
More detailed checking of the internet connection. This MR adds a callback to NetworkCallback's onCapabilitiesChanged() and verifies if the changed NetworkCapabilities indicate a working uplink connection. This change allows to register temporary internet connection losses (e.g. the WIFI router looses its internet connection) while the app stays on the same network.
The MR doesn't fix the detection of tor being censored in a network as described in #110.