bootanimation: Limit the PNGs to 250 colors

Some systems seem to have with the convert command as-is:
convert: Cannot write PNG8 or color-type 3; colormap is NULL

Because we are writing PNG8, there area a max of 256 colors total.
Explicitly telling convert to stay under the max fixes the issue.

Change-Id: I595fb4503396ca20226ea76bf7b15ed9878752fd