Fix janky scrolling for Gestures Settings.

1. Move the media player initialization/cleanup to construstor and
onDetach() of GesturePreference correspondingly, so that it's only
done once instead of every time when a new texture surface is available.
2. Pause the animation video when user scrolls.

Bug: 29790087
Change-Id: I51e6a787f9f2964b6bfc5288f7f5af801ccecc31