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/include/neteq.h b/webrtc/modules/audio_coding/neteq/include/neteq.h
index 5b52424..89b0c54 100644
--- a/webrtc/modules/audio_coding/neteq/include/neteq.h
+++ b/webrtc/modules/audio_coding/neteq/include/neteq.h
@@ -246,7 +246,7 @@
 
   // Returns the RTP timestamp for the last sample delivered by GetAudio().
   // The return value will be empty if no valid timestamp is available.
-  virtual rtc::Optional<uint32_t> GetPlayoutTimestamp() = 0;
+  virtual rtc::Optional<uint32_t> GetPlayoutTimestamp() const = 0;
 
   // Returns the sample rate in Hz of the audio produced in the last GetAudio
   // call. If GetAudio has not been called yet, the configured sample rate