Issue 3005: Decrease cpu usage for charging animation to nearly 0

A user pointed out 10-20% cpu usage for battery charging animation
on lowest possible cpu frequency. I optimized this a bit.

- Using an array to cache the mini icons now on creation
- So no more cpu overhead for updating the mini icon frame (former
  usage of temporary bitmaps and involved garbage collection)
- Using half the animation frames, since only every 2 frame (20%)
  the icon for battery actually changes

Change-Id: I510c223a07fa4da82ff4eb866bf0e3a2cca33dc1