HLS Refactor: Flatten createPeriod_() logic

Our previous approach:
For every variant tag, find the associated media tags(with the same
group id), create stream info for each media tag, and then create stream
info for variant tag based on that, and create variants.
This approach has duplicate calls for the media tags with the same group
id to create stream infos.

New approach:
1. Get codecs from variant tags
2. Create stream info from each media tag (with the codecs from variant
tags)
3. Create stream info for each variant tag, and then create variants.
This creates stream info for each media tag once, and avoids duplicate
calls to guess the codecs.

Issue shaka-project#1558
Closes shaka-project#2066

Change-Id: Icdf3133b1b15d1baf60624ff449e0a61fdb78d8a