store throughput in localStorage, not bitrate of last quality

Created by: spiterikevin

Storing the bitrate of one of the available representations can lead to excessive rounding down (#2124 (closed)) when different video streams are available for the same origin.

Also some ABR rule other than ThroughputRule can cause a drop in quality. E.g. if a live streaming session has a very shorter buffer, then InsufficientBufferRule will be conservative and limit the quality to a bitrate lower than the throughput. In such a case we want the throughput and not the representation bitrate to be stored in localStorage.