More reliable ALR detection

Previously AlrDetector was measuring amount of data sent in each 100ms
interval and would enter ALR mode after 5 consecutive intervals when
average bandwidth usage doesn't exceed 30% of the current estimate
estimate. This meant that an application that uses only slightely more
than 6% of total bandwidth may stay out of ALR mode, e.g. if it sends
a frame of size BW*30ms every 0.5 seconds. 100ms is too short interval
to average over, particularly when frame-rate falls below 10fps.

With this change AlrDetector averages BW usage over last 500ms. It then
enters ALR state when usage falls below 30% and exits it when usage
exceeds 50%.

BUG=webrtc:6332
R=philipel@webrtc.org, stefan@webrtc.org

Review URL: https://codereview.webrtc.org/2503643003 .

Cr-Commit-Position: refs/heads/master@{#15109}