Add the defaultNetwork element to the netstats.proto.

This is in its own commit because AOSP has diverged from internal
master.

Bug: 35142602
Test: builds, boots
Change-Id: I75fc85b961bc919e3d6c2adce6d6861c6f2c8379

core/java/android/net/NetworkIdentity.java

@@ -130,6 +130,7 @@ public class NetworkIdentity implements Comparable<NetworkIdentity> {
         proto.write(NetworkIdentityProto.NETWORK_ID, mNetworkId);
         proto.write(NetworkIdentityProto.ROAMING, mRoaming);
         proto.write(NetworkIdentityProto.METERED, mMetered);
+        proto.write(NetworkIdentityProto.DEFAULT_NETWORK, mDefaultNetwork);
 
         proto.end(start);
     }