PhaseBeam: Tri-phase the electron beam

This commit fixes Google's derps in PhaseBeam wallpaper:
- The bounds of the particles are fixed
- The wallpaper now have a nice scroll animation when
  you scroll wallpapers. Google disabled this because
  originally, the wallpaper only updates every 66ms. This
  caused the WP to be super laggy when scrolling. To
  circumvent this issue without melting your battery
  down, we offer a nice speed bump when the user is
  scrolling, so he doesn't notice that the wallpaper
  isn't updating at 15 fps (60 fps only when scrolling).

Change-Id: I69ae83fa454890aec5269dafae112057b88be2ab