Add more NetEq information to NetEqState.

Some important NetEq information was not available in NetEqState, which
meant it was not available on the API. This CL adds additional
information.

Bug: webrtc:9667
Change-Id: I702707c7d60472f488047d48fb286f839c5608dc
Reviewed-on: https://webrtc-review.googlesource.com/c/102300
Reviewed-by: Minyue Li <minyue@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Ivo Creusen <ivoc@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24985}

modules/audio_coding/neteq/packet_buffer.cc

@@ -91,6 +91,7 @@ int PacketBuffer::InsertPacket(Packet&& packet, StatisticsCalculator* stats) {
   if (buffer_.size() >= max_number_of_packets_) {
     // Buffer is full. Flush it.
     Flush();
+    stats->FlushedPacketBuffer();
     RTC_LOG(LS_WARNING) << "Packet buffer flushed";
     return_val = kFlushed;
   }