Remove extra handler#post inside setIncludeTestInterfaces

This was (mistakenly?) added in aosp/1267639 patchset 2; however, there
does not seem to be any apparent reason to post
trackAvailableInterfaces() to the back of the handler thread. All
operations that precede trackAvailableInterfaces() are synchronous.

Test: atest EthernetManagerTest
Change-Id: Ic8e888ce5d8909c4b7a00e26c54286c454ff3e75

service-t/src/com/android/server/ethernet/EthernetTracker.java

@@ -460,7 +460,7 @@ public class EthernetTracker {
             if (!include) {
                 removeTestData();
             }
-            mHandler.post(() -> trackAvailableInterfaces());
+            trackAvailableInterfaces();
         });
     }