Let NetEq use the PLC output from a decoder

This change enables NetEq to use the packet concealment audio (aka
PLC) produced by a decoder. The change also includes a new API to the
AudioDecoder interface, which lets the decoder implementation generate
and deliver concealment audio.

Bug: webrtc:9180
Change-Id: Icaacebccf645d4694b0d2d6310f6f2c7132881c4
Reviewed-on: https://webrtc-review.googlesource.com/96340
Commit-Queue: Henrik Lundin <henrik.lundin@webrtc.org>
Reviewed-by: Minyue Li <minyue@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24738}

modules/audio_coding/neteq/sync_buffer.h

@@ -13,6 +13,7 @@
 
 #include "api/audio/audio_frame.h"
 #include "modules/audio_coding/neteq/audio_multi_vector.h"
+#include "rtc_base/buffer.h"
 #include "rtc_base/constructormagic.h"
 
 namespace webrtc {
@@ -34,6 +35,9 @@ class SyncBuffer : public AudioMultiVector {
   // the move of the beginning of "future" data.
   void PushBack(const AudioMultiVector& append_this) override;
 
+  // Like PushBack, but reads the samples channel-interleaved from the input.
+  void PushBackInterleaved(const rtc::BufferT<int16_t>& append_this);
+
   // Adds |length| zeros to the beginning of each channel. Removes
   // the same number of samples from the end of the SyncBuffer, to
   // maintain a constant buffer size. The |next_index_| is updated to reflect