[Chromecast] Set focus type for communications.

Per device-id volume limits mean the content type exception is no longer
needed.
(https://chromium-review.googlesource.com/c/chromium/src/+/2071483)

Instead, the new focus type can be used to ensure the correct ducking
behavior:
(https://chromium-review.googlesource.com/c/chromium/src/+/2080583)

Bug: internal 144181483
Change-Id: I2ff83d18d142763f39ae6ea5ff0142b47142c9f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2088836Reviewed-by: Kenneth MacKay <kmackay@chromium.org>
Commit-Queue: Kenneth MacKay <kmackay@chromium.org>
Auto-Submit: Adam Whiteside <awhiteside@google.com>
Cr-Commit-Position: refs/heads/master@{#747314}

chromecast/media/audio/cast_audio_output_stream.cc

@@ -165,7 +165,8 @@ void CastAudioOutputStream::MixerServiceWrapper::Start(
   DCHECK_CALLED_ON_VALID_THREAD(io_thread_checker_);
 
   mixer_service::OutputStreamParams params;
-  params.set_content_type(ConvertContentType(GetContentType(device_id_)));
+  params.set_content_type(mixer_service::CONTENT_TYPE_MEDIA);
+  params.set_focus_type(ConvertContentType(GetContentType(device_id_)));
   params.set_device_id(device_id_);
   params.set_stream_type(
       mixer_service::OutputStreamParams::STREAM_TYPE_DEFAULT);