Keep muted audio frames muted when resampling

ResamplerHelper resampled muted frames as well and wrote the output
through mutable_data(), which clears the muted flag. When resampling,
ChannelReceive therefore returned muted NetEq output as kNormal, so the
mixer mixed silence instead of skipping the source, and the frames
weren't counted in decoded_muted_output.

- ResamplerHelper keeps a muted frame that follows a muted frame muted
  and doesn't resample it. The first muted frame after audio is still
  resampled and comes out unmuted, since it holds the resampler's tail
  of that audio. After muted frames, the resampler is primed with
  silence.
- ChannelReceive counts decoded_muted_output from NetEq's muted state
  before resampling.

Bug: none
Change-Id: I8bb8c5572911c172239005038554feacbb8f87cb
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/506980
Reviewed-by: Jakob Ivarsson‎ <jakobi@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48827}