Fix missing ->IN_SYNC transition.
Cap AudioTrack latencies at 10 seconds and log a warning if too large.
Throw a checked exception rather than unchecked one.

So that we actually catch it, rather than having the process crash!
Allow relaxation of TTML validity requirement when parsing subtitles.
Smoother playback #1.

Propagate elapsedRealtimeUs to the video renderer. This allows
the renderer to calculate and adjust for the elapsed time since
the start of the current rendering loop. Typically this is <2ms,
but there situations where it can go higher (normally when the
video renderer ends up processing more than 1 output buffer in
a single loop).

Also made variable naming more consistent throughout the package.
Treat "no chunk to load yet" in the same way as finished.

The key change here is that nextLoadPositionUs is set to -1
if we're not loading but don't have a next chunk ready to
load. This ensures that "missing chunks" in one stream don't
prevent chunks in another stream from loading. This occurs
in SmoothStreaming with TTML subtitles, where the chunks are
sparse.