Although FRAME-RATE is optional per HLS specification, but per HLS
Authoring Specification for Apple Devices (https://apple.co/30n90DC):
Each EXT-X-STREAM-INF tag MUST have a FRAME-RATE attribute.
Also, iOS and TVos refuse to play the content if VIDEO-RANGE is
present but FRAME-RATE is missing.
Issue #632, #634.
Change-Id: Ica282f023a7e8538f7b506094e2286840cf5c193
Instead, caclulating average bandwidth by dividing the sum of the
sizes of every segment by the sum of the durations of every segment.
This aligns with the requirement in HLS spec:
https://tools.ietf.org/html/draft-pantos-http-live-streaming-23 4.1.
BandwidthEstimator is also simplified to handle all blocks only.
Fixes#361
Change-Id: I89e7d415a841f4d4048f199de8dae7ffa250467b
KongQun Yang