SDK / exoplayer

This was a change suggested by @ojw28 to allow adpative track selection to work with IFrame only `Variant` as any other adaptive seleted `Track` in the `TrackGroup`.  This actually works quite well with the exceptions that:

1. IFrame only tracks do not contain the same Rendition Group references as other non-iframe only varaints (so track selection should disable these tracks even in other non-video renderers.
2. Adapting to iFrame tracks only based on bandwidth might not be so good (as often larger spacial resolution iFrame track might have higher bitrate then lower spacial resolution non-iframe only track).

This change includes one proposed workaround in the AdaptiveTrackSelection class (only use iFrame for higher playback speeds).