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 604dd40cf077f42c2d4b6ff80ff41d89cfbcacee)