Network notifications: revamp keying scheme

This patch changes the (tag: String, id: Int) keying scheme for network
notifications so that TRON notification counters can count network
related notifications unambiguously.

TRON instruments all notifications shown for package "android" as well
as user interactions with these Notifications. These counters are
grouped by id. However the network notifications ("no internet" dialog,
"captive portal sign in" dialog, ...) use a static tag and a dynamic id
for keying notifications, preventing the counters to correctly
aggregate. In addition there is also the risk of collision with other
SystemUi notification ids not managed by NetworkNotificationManager.

In order to make the TRON counters useful for network notifications,
the id is now encoding the network notification type in a stable way
while the tag is used to uniquely identify network notifications.

Test: change covered by previously added new unit tests.
Bug: 32198726
Bug: 33030620
Change-Id: I1201d1d4fab8183dd83449c2afb2c35cfece883f