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
Merged-In: Ib8658601e8d4dc6c41b335ab7dd8caa0cccd9531
Merged-In: I4432f66067ea1ab02e1d2dfe42530bcdafa52df6
Merged-In: I74631b0bfd14daf18a1641ed7f2ec323d636ebbf
Merged-In: I73cc879e910d503946facdba498b300337f349fd
Merged-In: Ieed9e3e7755e0c5f89dc41ef66f47d4dbf4c66a9
Merged-In: I0aa590170f1bd4c37175c7e35e54d52f1fb21347

(cherry picked from commit 5fcd050e)

Change-Id: I41675768ab59e9b23ca4275edf297b82595e5730