Refactor NetworkCallback listener to correctly track connectivity under VPN

[valldrac]

Previously, the listener could incorrectly report Internet access as available when it wasn't, or unavailable when it actually was.

Changes:

• Remove NET_CAPABILITY_NOT_VPN to correctly track VPN networks.
• Track connectivity via a restartable coroutine Flow.
• Handle VPN state transitions when kill-switch is enabled.
• Force connection attempt on push notification, regardless of reported network state.

We already track all networks individually rather than using a single state, unlike upstream (2dae3cc, 7859ca3).

Fixes #352, fixes #663

[p1gp1g]

Ok, I've commented a few things to improve the kotlin flows.

One of the most important comment is about onLost.

But the problem exists even if you always close the coroutine onLost. That's because Molly may start when the VPN is considered connected but the device doesn't have any transport network available: we get networks = (123 to NetworkState(validated = true, blocked = false, onVpn = true)) and conclude isOnline = true

EDIT: in this case bestNetworkState changed validated to false => that's OK, unless there is another weird bug

[p1gp1g]

There is a repetitive check, else, it looks good to me

[valldrac]

Fixed the unnecessary check. Also renamed the class to InternetConnectivityMonitor, it better describes its purpose and avoids merge conflicts with upstream.