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 604dd40cf077f42c2d4b6ff80ff41d89cfbcacee)

core/java/android/net/ConnectivityDiagnosticsManager.java

@@ -659,8 +659,8 @@ public class ConnectivityDiagnosticsManager {
     public abstract static class ConnectivityDiagnosticsCallback {
         /**
          * Called when the platform completes a data connectivity check. This will also be invoked
-         * immediately upon registration with the latest report, if a report has already been
-         * generated for this network.
+         * immediately upon registration for each network matching the request with the latest
+         * report, if a report has already been generated for that network.
          *
          * <p>The Network specified in the ConnectivityReport may not be active any more when this
          * method is invoked.

services/core/java/com/android/server/connectivity/NetworkAgentInfo.java

@@ -244,8 +244,9 @@ public class NetworkAgentInfo implements Comparable<NetworkAgentInfo> {
     // How many of the satisfied requests are of type BACKGROUND_REQUEST.
     private int mNumBackgroundNetworkRequests = 0;
 
-    // The last ConnectivityReport made available for this network.
-    private ConnectivityReport mConnectivityReport;
+    // The last ConnectivityReport made available for this network. This value is only null before a
+    // report is generated. Once non-null, it will never be null again.
+    @Nullable private ConnectivityReport mConnectivityReport;
 
     public final Messenger messenger;
     public final AsyncChannel asyncChannel;