[firebase_messaging] fix crash when func deleteInstanceID return result in incorrect thread

packages/firebase_messaging/android/src/main/java/io/flutter/plugins/firebasemessaging/FirebaseMessagingPlugin.java

@@ -9,6 +9,8 @@
 import android.content.Intent;
 import android.content.IntentFilter;
 import android.os.Bundle;
+import android.os.Handler;
+import android.os.Looper;
 import android.util.Log;
 import androidx.annotation.NonNull;
 import androidx.localbroadcastmanager.content.LocalBroadcastManager;
@@ -148,14 +150,24 @@ public void onComplete(@NonNull Task<InstanceIdResult> task) {
                 public void run() {
                   try {
                     FirebaseInstanceId.getInstance().deleteInstanceId();
-                    result.success(true);
+                    new Handler(Looper.getMainLooper()).post(new Runnable() {
+                      @Override
+                      public void run() {
+                        result.success(true);
+                      }
+                    });
                   } catch (IOException ex) {
                     Log.e(TAG, "deleteInstanceID, error:", ex);
-                    result.success(false);
+                    new Handler(Looper.getMainLooper()).post(new Runnable() {
+                      @Override
+                      public void run() {
+                        result.success(false);
+                      }
+                    });
                   }
                 }
               })
-          .start();
+              .start();
     } else if ("autoInitEnabled".equals(call.method)) {
       result.success(FirebaseMessaging.getInstance().isAutoInitEnabled());
     } else if ("setAutoInitEnabled".equals(call.method)) {