LowpanCommissioningSession: Add `static abstract` keywords

These keywords were accidentally left off of the original declaration.

Bug: b/63645461
Change-Id: I1cadcd68831377ee1d1059f645f7178b7e5c742e
Tests: Verified by being able to successfully compile
	I46667d4ea337463a35a9dd31fb00f7e130553d53

lowpan/java/android/net/lowpan/LowpanCommissioningSession.java

@@ -40,7 +40,7 @@ public abstract class LowpanCommissioningSession {
      * @hide
      */
     // @SystemApi
-    public class Callback {
+    public static abstract class Callback {
         public void onReceiveFromCommissioner(@NonNull byte[] packet) {};
 
         public void onClosed() {};