Prompt if a network without an Internet connection is selected

When a network is explicitlySelected, keep it connected but do
not automatically switch to it. Instead, attempt to validate it,
and if 8 seconds have passed and the network is not yet
validated, prompt the user asking whether to switch to it anyway.

Bug: 20081183
Change-Id: I03a8459eb39979e3dc8e94662b85a44605dd7e69