Small refactor in DartMessenger.java to use method reference (#178584)

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Author: AbdeMohlbi

engine/src/flutter/shell/platform/android/io/flutter/embedding/engine/dart/DartMessenger.java

@@ -158,10 +158,7 @@ static class SerialTaskQueue implements DartMessengerTaskQueue {
     @Override
     public void dispatch(@NonNull Runnable runnable) {
       queue.add(runnable);
-      executor.execute(
-          () -> {
-            flush();
-          });
+      executor.execute(this::flush);
     }
 
     private void flush() {
@@ -176,10 +173,7 @@ private void flush() {
           isRunning.set(false);
           if (!queue.isEmpty()) {
             // Schedule the next event.
-            executor.execute(
-                () -> {
-                  flush();
-                });
+            executor.execute(this::flush);
           }
         }
       }