Decoupling audio_device from Obj-C code

The goal of this CL is to separate Obj-C/Obj-C++ code from targets which have
also C++ code (see https://bugs.chromium.org/p/webrtc/issues/detail?id=7743
for more information).

To achieve this we have created 2 targets (audio_device_ios_objc and
audio_device_generic) and audio_device will act as a proxy between these targets
(this way we can avoid a circular dependency between audio_device_generic and
audio_device_ios_objc).

BUG=webrtc:7743

Review-Url: https://codereview.webrtc.org/2991343002
Cr-Commit-Position: refs/heads/master@{#19795}

webrtc/sdk/objc/Framework/Classes/Audio/RTCAudioSession+Private.h

@@ -8,7 +8,7 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
-#import "WebRTC/RTCAudioSession.h"
+#import "webrtc/sdk/objc/Framework/Headers/WebRTC/RTCAudioSession.h"
 
 #include <vector>