Add setting to AppRTCMobile for iOS, that can change capture resolution.

webrtc/examples/objc/AppRTCMobile/ios/ARDMediaConstraintsSettingsStore.h

+/*
+ *  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.
+ */
+
+#import <Foundation/Foundation.h>
+
+NS_ASSUME_NONNULL_BEGIN
+
+/**
+ * Light-weight persistent store for media constraints user settings.
+ *
+ * It will persist between application launches and application updates.
+ */
+@interface ARDMediaConstraintsSettingsStore : NSObject
+
+/**
+ * Returns current video resolution media constraint string stored in the store.
+ */
+- (nullable NSString *)videoResolutionConstraintsSetting;
+
+/**
+ * Stores the provided value as video resolution media constraint.
+ * @param value the string to be stored
+ */
+- (void)setVideoResolutionConstraintsSetting:(NSString *)value;
+
+@end
+NS_ASSUME_NONNULL_END