Fixes #220 Screen Mirroring Functionality

[sameerjj]

• modify sample/ar-cursor-placement code to have MediaRecorder functionality based on action button
  • because of the MediaPlayer state machine, I wasn't able to use sceneView.startRecording() directly as it wasn't possible to configure it correctly (setVideoSource, setOutputFile, setVideoEncoder had to be called in a very specific order)
• modify SceneView to use a more generic startMirroring() function and not try to do too much MediaPlayer related activity.
• render.copyFrame() has to be called between renderer.render(view) and renderer.endFrame() add screenMirror call there
• modify ScreenMirrorer to not depend on lifecycle but rather manual call from SceneView.

TODO could getLetterboxViewport be moved to startMirroring?

[ThomasGorisse]

Thanks @sameerjj