[Merge-148] Cherry pick "Move the NullVideoDecoder into a separate file and target."

Original change's description:
> Move the NullVideoDecoder into a separate file and target.
>
> This is necessary because the Chromium video decoder factory needs to
> be able to instantiate it for fallback purposes (e.g. Decode error
> causing the HW to no longer be available).
>
> Bug: chromium:500960863
> Change-Id: I724a5f28aa0a0a615972dd49eebde701c3b7ec90
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/462243
> Reviewed-by: Evan Shrubsole <eshr@webrtc.org>
> Commit-Queue: Henrik Boström <hbos@webrtc.org>
> Cr-Commit-Position: refs/heads/main@{#47366}

Bug: chromium:500960863
Change-Id: I724a5f28aa0a0a615972dd49eebde701c3b7ec90
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/464521
Reviewed-by: Ilya Nikolaevskiy <ilnik@webrtc.org>
Commit-Queue: Evan Shrubsole <eshr@webrtc.org>
Cr-Commit-Position: refs/branch-heads/7778@{#1}
Cr-Branched-From: ca896b7ffef011bbf6957c99d413c5aac602c99f-refs/heads/main@{#47319}