Fix NullPointerException happens in dumpCheckin

Currently dumpCheckin passes a null object to create
the contructor of NetworkStatsCollection.Key but the
constructor requires non-null object. Thus, it caused
the NPE in dumpCheckin. To fix this exception, create
an NetworkIdentitySet() object instead of using an
null object.

Bug: 225131008
Bug: 226539404
Test: dumpsys netstats --checkin is fine
      FrameworksNetTests
Change-Id: I7f2dadf0647b3f42e0f667d96291d2ae37e23faf

framework-t/src/android/net/NetworkIdentitySet.java

@@ -206,6 +206,7 @@ public class NetworkIdentitySet extends HashSet<NetworkIdentity> {
     public static int compare(@NonNull NetworkIdentitySet left, @NonNull NetworkIdentitySet right) {
         Objects.requireNonNull(left);
         Objects.requireNonNull(right);
+        if (left.isEmpty() && right.isEmpty()) return 0;
         if (left.isEmpty()) return -1;
         if (right.isEmpty()) return 1;
 

framework-t/src/android/net/NetworkStatsCollection.java

@@ -776,7 +776,7 @@ public class NetworkStatsCollection implements FileRotator.Reader, FileRotator.W
             if (!templateMatches(groupTemplate, key.ident)) continue;
             if (key.set >= NetworkStats.SET_DEBUG_START) continue;
 
-            final Key groupKey = new Key(null, key.uid, key.set, key.tag);
+            final Key groupKey = new Key(new NetworkIdentitySet(), key.uid, key.set, key.tag);
             NetworkStatsHistory groupHistory = grouped.get(groupKey);
             if (groupHistory == null) {
                 groupHistory = new NetworkStatsHistory(value.getBucketDuration());