[wpilib] Make Preferences Listener not depend on mutable fields (#8607)

The Listener installed by Preferences was referencing m_typePublisher which could be modified by a future call to setNetworkTableInstance(). Instead, reference a local.

Also made Topic.m_handle final, to guarantee that Topic.equals() is thread-safe, and still work after the publisher has been closed.

ntcore/src/main/java/edu/wpi/first/networktables/Topic.java

@@ -330,8 +330,8 @@ public class Topic {
   }
 
   /** NetworkTables instance. */
-  protected NetworkTableInstance m_inst;
+  protected final NetworkTableInstance m_inst;
 
   /** NetworkTables handle. */
-  protected int m_handle;
+  protected final int m_handle;
 }