Move stream_factory.mojom to media/mojo/mojom/.

In order to support offline speech recognition of microphone input,
we plan to add a method to SpeechRecognitionContext that takes a
StreamFactory. However, StreamFactory depends on media::mojom, so
adding StreamFactory to speech_recognition_service.mojom in media/
will create a circular dependency.

This change moves services/audio/public/mojom/stream_factory.mojom
to media/mojo/mojom/stream_factory.mojom to avoid that circular
dependency, and renames StreamFactory to AudioStreamFactory.

This change makes no functional changes.

Proposed addition to speech_recognition_service.mojom (follow-up):
```
// Allows audio to be captured from within the SpeechRecognitionService
// process, with results passed back to the
// SpeechRecognitionRecognizerClient.
BindAudioSourceRecognizer(
               pending_receiver<AudioSourceFetcher> fetcher_receiver,
               pending_remote<SpeechRecognitionRecognizerClient> client,
               pending_remote<media.mojom.AudioStreamFactory> factory)
               => (bool success);
```

See design at go/cros-dictation-with-soda (audio input header).

Bug: 1173135
Change-Id: Ifa286d628b8d4398b41f0b2daed704989ef5fe84
AX-Relnote: N/A
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2718486
Reviewed-by: Alex Gough <ajgo@chromium.org>
Reviewed-by: Jeroen Dhollander <jeroendh@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Evan Liu <evliu@google.com>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#859106}