NetEq/ACM: Refactor how packet waiting times are calculated

With this change, the aggregates for packet waiting times are
calculated in NetEq's StatisticsCalculator insead of in
AcmReceiver. This simplifies things somewhat, and avoids having to
copy the raw data on polling.

R=ivoc@webrtc.org, minyue@webrtc.org

Review URL: https://codereview.webrtc.org/1296633002 .

Cr-Commit-Position: refs/heads/master@{#9778}
7 files changed