charger: Ensure every generated image is 8-bit

minui supports only <= 8-bit PNGs.

On my machine battery_fail was being encoded
using 24-bits and charger was freaking out :(

Change-Id: I2f63548ba102f49485c43268389f1cdd5e50802d
Signed-off-by: Luca Stefani <luca.stefani.ge1@gmail.com>