SDK / exoplayer

This change merges the duties of FormatEvaluator into
TrackSelection classes, so that both the static and
dynamic parts of track selection are implemented in a
single place.

New feature: Demo app now allows you to enable random
adaptation in the track selection dialog.

Notes:

- It should be quite easy to allow application side
track blacklisting in addition to source side, as an
extension to this. That would effectively allow
applications to do seamless/deferred track selection
by creating a TrackSelection with all tracks enabled,
and then toggling the blacklist flags to select the
ones they want to be active.

- It should be trivial to implement format blacklisting
for DASH and SS as an extension to this. Will do in a
follow up CL.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=128707517