Lagging Behind control

[yuroyami]

ExoPlayer does not provide a good set of methods or interfaces to control videos' playback speed. However, a way to control speed should be implemented in order to port the following settings:

When the main user lags behind:

• Checkable option: Fast-forward when lagging behind (on by default on Desktop).
• Checkable option: Don't slow down or rewind others when lagging behind (off on Desktop, and is still experimental).

When others lag behind:

• Checkable option: Slow down on minor desync (On by default on Desktop)
• Checkable option: Rewind on major desync (Already implemented on Android, even rewind threshold is adjustable, but there has to be an option to disable it)