Add HLS quality support

[IvanStepanok]

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[sdaitzman]

I think this is an area where the engineering is a bit ahead of our design efforts, but this looks good to me based on the existing designs. iOS/Android video quality language should match. I think it should be "Lower data usage," or something similar.

For comparison, YouTube uses "Lower picture quality" for lowest-quality, and "Uses more data" for highest-quality. For downloads, the current EdX app uses "Smallest file size" for lowest-quality, and "Best quality" for highest-quality for mobile video downloads.

(Similar comment also posted on openedx/openedx-app-android#76)

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