m3u8: Parse and use AVERAGE-BANDWIDTH attribute if available

m3u8: Parse and use AVERAGE-BANDWIDTH attribute if available

The AVERAGE-BANDWIDTH attribute in the EXT-X-STREAM-INF tag represents
the average segment bit rate of the Variant Stream, while the BANDWIDTH
attribute represents the peak segment bit rate of the Variant Stream.
(https://tools.ietf.org/html/draft-pantos-http-live-streaming-23#section-4.3.4.2)
Using the average bit rate instead of the peak bit rate for variant switching
is more efficient and appropriate. Sometimes due to VBR encoding,
the BANDWIDTH may represent a value way above the average bit rate,
which could result to players not switching to that variant stream
 although network bandwidth is sufficiently available.

https://bugzilla.gnome.org/show_bug.cgi?id=790821