[media_preview] Fix tests broken by deferred device settings selection

This fixes two bugs that were breaking tests.

1) Only do deferred processing when the `MediaStreamType` is for
   device capture.
2) In OnStreamsGenerated, the stream device for audio/video can be
   nullopt if the permission wasn't granted. This seems to only be
   possible in tests, but there are a few tests that cause this
   state.

 Bug: 343505105

Change-Id: I855ee1b1b65c05c33ddfd32930b11dc9a1a719d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5581683
Commit-Queue: Bryant Chandler <bryantchandler@chromium.org>
Reviewed-by: Guido Urdaneta <guidou@chromium.org>
Reviewed-by: Bryant Chandler <bryantchandler@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1308272}