Move setting of AudioFrame::timestamp_ into NetEq

This was previously done in AcmReceiver, but belongs in NetEq where the
rest of the AudioFrame fields are populated.

BUG=webrtc:5669,webrtc:5607

Review URL: https://codereview.webrtc.org/1863993002

Cr-Commit-Position: refs/heads/master@{#12265}
diff --git a/webrtc/modules/audio_coding/neteq/neteq_impl.h b/webrtc/modules/audio_coding/neteq/neteq_impl.h
index 737efd4..75055a7 100644
--- a/webrtc/modules/audio_coding/neteq/neteq_impl.h
+++ b/webrtc/modules/audio_coding/neteq/neteq_impl.h
@@ -160,7 +160,7 @@
   // Disables post-decode VAD.
   void DisableVad() override;
 
-  rtc::Optional<uint32_t> GetPlayoutTimestamp() override;
+  rtc::Optional<uint32_t> GetPlayoutTimestamp() const override;
 
   int last_output_sample_rate_hz() const override;