When no Internet access is detected, display a notification first.

Previously, we displayed a dialog on top of whatever the user was
doing. Instead, display a notification that can take the user to
the dialog.

Because the notification is less intrusive, also enable it even
if the network without Internet access is already the system
default network. Previously we did not do this because in that
case the user might not have other useful options, and thus a
dialog would have been too intrusive.

Bug: 20081183
Bug: 20538448
Change-Id: I677be238d675c13a13dc0bae2dbb37cbe4f52cb6