Support delayed AudioUnit initialization.

Applications can choose to decide when to give up control of the
AVAudioSession to WebRTC. Otherwise, behavior should be
unchanged.

Adds a toggle to AppRTCDemo so developers can see the different
paths.

BUG=
R=haysc@webrtc.org

Review URL: https://codereview.webrtc.org/1822543002 .

Cr-Commit-Position: refs/heads/master@{#12080}

webrtc/examples/objc/AppRTCDemo/ios/ARDMainViewController.h

@@ -11,7 +11,4 @@
 #import <UIKit/UIKit.h>
 
 @interface ARDMainViewController : UIViewController
-
-- (void)applicationWillResignActive:(UIApplication *)application;
-
 @end