SDK / exoplayer

When ChunkSource implementations implement multi-track for DASH and SS,
format selection will move inside of ChunkSource. If we, for example, fail
to query the decoder to determine which tracks are playable, we need an
opportunity to fail (i.e. say we're not prepared, so that maybeThrowError
is called, from which we can throw).

This may go away in the future if we remove the distinct preparation step
and treat tracks/formats as things that can change dynamically, but for now
this is what we have.

Issue #514.