Access UIApplication on main thread

Track UIApplication applicationState changes from a C++ class. Uses
NSNotificationCenter to access changes on the main thread and exposes
a local variable that can be checked from any thread.

This fixes a runtime warning on iOS 11 beta.

My Objective-C++ is a little rusty so please check if this follows
the conventions for C++ code in the project. It also changes the
interface exposed by RTCUIApplication.h, not sure if that has impact
on any public APIs that needs to be documented somewhere?

Bug: webrtc:7773
Change-Id: I9c8ba090ef9f28d812114026a906cef742192c39
Reviewed-on: https://chromium-review.googlesource.com/527442
Reviewed-by: Magnus Jedvert <magjed@webrtc.org>
Reviewed-by: Kári Tristan Helgason <kthelgason@webrtc.org>
Commit-Queue: Anders Carlsson <andersc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#18558}

webrtc/sdk/objc/Framework/Classes/Common/RTCUIApplicationStatusObserver.h

@@ -0,0 +1,23 @@
+/*
+ *  Copyright 2016 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#if defined(WEBRTC_IOS)
+
+#import <Foundation/Foundation.h>
+
+@interface RTCUIApplicationStatusObserver : NSObject
+
++ (instancetype)sharedInstance;
+
+- (BOOL)isApplicationActive;
+
+@end
+
+#endif  // WEBRTC_IOS