Add option to make sign-in notification ongoing

Add an overlay boolean that allows setting the SIGN_IN notification as
an ongoing notification.

This can be useful to make sure users can always easily find the
notification to sign in to a captive portal, as studies have found that
some users have a tendency to dismiss notifications before reading them.
At the same time the notification shade is generally too crowded, which
is what causes such behaviors in the first place, so this option is not
enabled by default and should generally not be enabled without proper
user studies or metrics.

Bug: 173171709
Test: atest NetworkNotificationManagerTest
Change-Id: Ic187d2a2b7e49ad152ea2aa35bb784864b97473c