Switched TimerTask for frame rendering to ScheduledThreadPoolExecutor

This changes the renderer from using `TimerTask` to schedule frame updates to using `ScheduledThreadPoolExecutor`. This may fix the stale and closed issue of MasDennis/Rajawali#630 but I am not certain. In either case I feel like this is a more robust way of scheduling the frame rate since everything in my experience has told me to avoid `TimerTask` like the plague.