remove pixelflinger from healthd

The minui library has changed to no longer use libpixelflinger.  It
also offers a more efficient way of storing and loading frames of
animation; use it to represent the battery-full state images.

Also removes the unused battery_charge.png image.

Bug: 12131110
Change-Id: I6cc8b19f36a806c99cda30cc47b5968daf6b333b