Add one frame (10 ms) of silence in APM output after unmuting

This CL adds one frame (10 ms) of silence in APM output after unmuting to mask
audio resulting from the turning on the processing that was deactivated
during the muting.

Bug: b/177830919
Change-Id: If44cfb0ef270dde839dcd3f0b98d1c91e81668dd
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/211343
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Commit-Queue: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33454}

modules/audio_processing/audio_processing_impl.h

@@ -433,6 +433,7 @@ class AudioProcessingImpl : public AudioProcessing {
     ~ApmCaptureState();
     bool was_stream_delay_set;
     bool capture_output_used;
+    bool capture_output_used_last_frame;
     bool key_pressed;
     std::unique_ptr<AudioBuffer> capture_audio;
     std::unique_ptr<AudioBuffer> capture_fullband_audio;