Add sleep to wait for the next event in the main loop properly

[HidenoriMatsubayashi]

Currently, there isn't any sleep to wait for the next event in the main loop properly. Therefore, it is high CPU usage.

main.cc

  // Main loop.
  while (flutter_controller->view()->DispatchEvent()) {
    flutter_controller->engine()->ProcessMessages();
  }

[LefixBlue]

@HidenoriMatsubayashi

That's obviously the one I was talking about when I reported my test results on the raspberrypi-64 yocto build in #4

[HidenoriMatsubayashi]

Yes, I think so. Thank you for your report!

[HidenoriMatsubayashi]

I've fixed this issue #45. Thank you so much!