Make RTCAudioSession accessible to Swift.

This is done by:
1. removing the <vector> include from RTCAudioSession+Private,
2. creating a audio_session_objc package that excludes the RTCNativeAudioSessionDelegateAdapter class.

Bug: webrtc:11237
Change-Id: I36c86542a19e3244456fd164d908563b1435de29
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/163900
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Reviewed-by: Chuck Hays <haysc@webrtc.org>
Reviewed-by: Henrik Andreassson <henrika@webrtc.org>
Commit-Queue: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30177}

sdk/objc/unittests/RTCAudioSessionTest.mm

@@ -11,6 +11,8 @@
 #import <Foundation/Foundation.h>
 #import <OCMock/OCMock.h>
 
+#include <vector>
+
 #include "rtc_base/gunit.h"
 
 #import "components/audio/RTCAudioSession+Private.h"
@@ -20,6 +22,8 @@
 
 @interface RTCAudioSession (UnitTesting)
 
+@property(nonatomic, readonly) std::vector<__weak id<RTCAudioSessionDelegate> > delegates;
+
 - (instancetype)initWithAudioSession:(id)audioSession;
 
 @end