JitterEstimator: rename and reorder constants.

The constants are reordered to match the order they are used
when a sample is inserted into the filter. Some of the constants
are renamed to better describe their usage. No functional changes
are intended. Future CLs will add configurability to some of these
constants.

Some basic unit tests are also added.

Bug: webrtc:14151
Change-Id: I731a9cad3d8aeab06ccfa7d212cd160a2d2da27b
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/274122
Commit-Queue: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Philip Eliasson <philipel@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#38019}

modules/video_coding/timing/jitter_estimator.h

@@ -61,11 +61,6 @@ class JitterEstimator {
   //          - rtt          : Round trip time.
   void UpdateRtt(TimeDelta rtt);
 
-  // A constant describing the delay from the jitter buffer to the delay on the
-  // receiving side which is not accounted for by the jitter buffer nor the
-  // decoding delay estimate.
-  static constexpr TimeDelta OPERATING_SYSTEM_JITTER = TimeDelta::Millis(10);
-
  private:
   // Updates the random jitter estimate, i.e. the variance of the time
   // deviations from the line given by the Kalman filter.