Clarify comments for Connectivity Diagnostics reports.

Clarify when
ConnectivityDiagnosticsCallback#onConnectivityReportAvailable will be
invoked. Clarify when NetworkAgentInfo#mConnectivityReport will be null
vs non-null.

Bug: 147849853
Test: atest FrameworksNetTests
Change-Id: I748bd9ded72a34d89f13bd4362d6d4da62b910b8
Merged-In: I748bd9ded72a34d89f13bd4362d6d4da62b910b8
(cherry picked from commit 604dd40c)