Notify NetworkStats only when interfaces changed in updateLinkProperties

In ConnectivityService, updateLinkProperties calls NetworkStats
even though the ip address or interface name has not changed and
only the tcp buffer size has changed. This is noisy and could
be problematic when RAT change occurs frequently, since when
RAT changes tcp buffer size configuration also changes.

This CL also fixes a wrong nullability annotation where the oldLp
of updateLinkProperties could be null when updateNetworkInfo
is called.

Test: atest ConnectivityServiceTest#testStatsIfacesChanged
Fix: 232048480
Change-Id: Ic226eb4a8aa1f38cba293510813f1cb55f0ea658

tests/unit/java/com/android/server/ConnectivityServiceTest.java

@@ -7109,6 +7109,15 @@ public class ConnectivityServiceTest {
         expectNotifyNetworkStatus(onlyCell, MOBILE_IFNAME);
         reset(mStatsManager);
 
+        // Verify change fields other than interfaces does not trigger a notification to NSS.
+        cellLp.addLinkAddress(new LinkAddress("192.0.2.4/24"));
+        cellLp.addRoute(new RouteInfo((IpPrefix) null, InetAddress.getByName("192.0.2.4"),
+                MOBILE_IFNAME));
+        cellLp.setDnsServers(List.of(InetAddress.getAllByName("8.8.8.8")));
+        mCellNetworkAgent.sendLinkProperties(cellLp);
+        verifyNoMoreInteractions(mStatsManager);
+        reset(mStatsManager);
+
         // Default network switch should update ifaces.
         mWiFiNetworkAgent.connect(false);
         mWiFiNetworkAgent.sendLinkProperties(wifiLp);