[Thread] Let TunInterfaceController.getLinkProperties() return a copy of LinkProperties

This CL avoids accidental edits to the `mLinkProperties` field of TunInterfaceController.

Change-Id: Ie696412c0593cece2fddcd6bc097db2829ef16e7
diff --git a/thread/service/java/com/android/server/thread/TunInterfaceController.java b/thread/service/java/com/android/server/thread/TunInterfaceController.java
index 85a0371..3bff9c6 100644
--- a/thread/service/java/com/android/server/thread/TunInterfaceController.java
+++ b/thread/service/java/com/android/server/thread/TunInterfaceController.java
@@ -93,7 +93,7 @@
 
     /** Returns link properties of the Thread TUN interface. */
     public LinkProperties getLinkProperties() {
-        return mLinkProperties;
+        return new LinkProperties(mLinkProperties);
     }
 
     /**