Use heads-up notifications when selecting networks with no Internet

Currently, when connecting to a network that has a captive portal
or has no Internet access, we display a regular notification.
Because this notification is easy to miss, switch to using a
heads-up notification if the user just manually selected the
network. If the system connects automatically, continue to use a
regular notification.

Bug: 20081183
Change-Id: I7a988b2bddfe898a0d2607ad85a04b227d678469