Update iOS AppRTCDemo to use level controller.

NOTRY=True
BUG=

Review-Url: https://codereview.webrtc.org/2294913003
Cr-Commit-Position: refs/heads/master@{#13977}

webrtc/examples/objc/AppRTCDemo/ARDAppClient+Internal.h

@@ -45,6 +45,7 @@
 @property(nonatomic, readonly) BOOL isAudioOnly;
 @property(nonatomic, readonly) BOOL shouldMakeAecDump;
 @property(nonatomic, assign) BOOL isAecDumpActive;
+@property(nonatomic, readonly) BOOL shouldUseLevelControl;
 
 @property(nonatomic, strong)
     RTCMediaConstraints *defaultPeerConnectionConstraints;