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.
- Add constants class. Currently housing a single lonely variable,
which is used generally throughout the library, and so no longer
nicely fits into a specific class.
- Rename a few other constants to add clear units.
- Made minor tweak to ExoPlayer documentation.
