NetworkNotificationManager: correctly handle existing notifications

This patch corrects a regression added by commit fb2609d3 that did
not take into account the case of multiple notifications shown for a
single network id. Given how network notifications are triggered, it can
happen that NO_INTERNET and SIGN_IN notifications are both triggered for
the same network when captive portal detection is slow.

Contrary to the situation before commit fb2609d3, a notification
priority order is introduced so that SIGN_IN always overrides
NO_INTERNET, and NO_INTERNET is ignored if SIGN_IN is already present.

Bug: 63676954
Bug: 62503737
Test: runtest frameworks-net, added new unit tests
Change-Id: Ib8658601e8d4dc6c41b335ab7dd8caa0cccd9531