Re-notify if the network goes to PARTIAL.

If NetworkMonitor detects partial connectivity before
EVENT_PROMPT_UNVALIDATED arrives, show the partial
connectivity notification immediately. Re-notify
partial connectivity silently if no internet
notification already there.

Bug: 130683832
Bug: 130766237
Test: atest com.android.server.ConnectivityServiceTest
Change-Id: I7d4eddc643ec795c3961097dc1bdd314d168f6c7