review fixes

packages/shared_preferences/android/src/main/java/io/flutter/plugins/sharedpreferences/SharedPreferencesPlugin.java

@@ -29,12 +29,17 @@ public void onAttachedToEngine(@NonNull FlutterPlugin.FlutterPluginBinding bindi
 
   @Override
   public void onDetachedFromEngine(@NonNull FlutterPlugin.FlutterPluginBinding binding) {
-    channel.setMethodCallHandler(null);
+    teardownChannel();
   }
 
   private void setupChannel(BinaryMessenger messenger, Context context) {
     channel = new MethodChannel(messenger, CHANNEL_NAME);
     MethodCallHandlerImpl handler = new MethodCallHandlerImpl(context);
     channel.setMethodCallHandler(handler);
   }
+
+  private void teardownChannel() {
+    channel.setMethodCallHandler(null);
+    channel = null;
+  }
 }

packages/shared_preferences/example/android/app/src/main/AndroidManifest.xml

@@ -12,7 +12,6 @@
                   android:windowSoftInputMode="adjustResize">
         </activity>
         <activity android:name=".MainActivity"
-            android:launchMode="singleTop"
             android:theme="@android:style/Theme.Black.NoTitleBar"
             android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection"
             android:hardwareAccelerated="true"