Move xplat/sonar/xplat/Sonar* to Flipper*

Summary:
Part of the Sonar -> Flipper rename
Intentionally left externally visible buck target as Sonar for now to minimize diff size.

Reviewed By: passy

Differential Revision: D9871684

fbshipit-source-id: 6926eb62c578a05cd895745c75ed7da3cfe965b5

xplat/Flipper/SonarStateUpdateListener.h

@@ -0,0 +1,14 @@
+/*
+ *  Copyright (c) 2018-present, Facebook, Inc.
+ *
+ *  This source code is licensed under the MIT license found in the LICENSE
+ *  file in the root directory of this source tree.
+ *
+ */
+
+#pragma once
+
+class SonarStateUpdateListener {
+ public:
+  virtual void onUpdate() = 0;
+};