Add getters to UnderlyingNetworkInfo

Address API review feedback, add getters to UnderlyingNetworkInfo
instead of exposing fields.

Instead of wasting memory by converting this into an array, have
migrateTun take a List<String>. In turn, tunAdjustmentInit should
also take a List<String>.

(cherry picked from ag/14211075)
Bug: 183972554
Test: atest android.net.UnderlyingNetworkInfoTest
Merged-In: Id59744097208d91298a25ef110ade91a9cf291a1
Change-Id: Id59744097208d91298a25ef110ade91a9cf291a1

services/core/java/com/android/server/net/NetworkStatsFactory.java

@@ -382,8 +382,8 @@ public class NetworkStatsFactory {
 
         // Migrate data usage over a VPN to the TUN network.
         for (UnderlyingNetworkInfo info : vpnArray) {
-            delta.migrateTun(info.ownerUid, info.iface,
-                    info.underlyingIfaces.toArray(new String[0]));
+            delta.migrateTun(info.getOwnerUid(), info.getIface(),
+                    info.getUnderlyingIfaces());
             // Filter out debug entries as that may lead to over counting.
             delta.filterDebugEntries();
         }