Only show "Connected" note after opening portal

The "Connected" notification would be shown every time a network
validates after being identified as a captive portal. This causes issues
on networks that have auto-login mechanisms, as a high priority
notification would be shown even though the user was not interacting
with the phone.

The "Connected" notification is intended to confirm to the user that
they successfuly logged in (manually), so only show it after the user
opens the portal on the network.

Bug: 134124044
Test: Flashed, connected to portal: notification shown
      Opened portal from command line + revalidate: no notification
      Tests passing with change, failing without

Change-Id: I89e7a7f49019bd7e4686712c56e00bd786eb3ef3