Use RTCAudioSessionDelegateAdapter in AudioDeviceIOS.

Part 3 of refactor. Also:
- better weak pointer delegate storage + tests
- we now ignore route changes when we're interrupted
- fixed bug where preferred sample rate wasn't set if audio session
   wasn't active

BUG=

Review URL: https://codereview.webrtc.org/1796983004

Cr-Commit-Position: refs/heads/master@{#12007}

webrtc/examples/objc/AppRTCDemo/ios/Info.plist

@@ -58,6 +58,7 @@
   </array>
   <key>UIBackgroundModes</key>
   <array>
+    <string>audio</string>
     <string>voip</string>
   </array>
   <key>UILaunchImages</key>