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

bootanimation/generate-bootanimation.sh

@@ -26,7 +26,7 @@ for part_cnt in 0 1 2 3 4
 do
     mkdir -p $ANDROID_PRODUCT_OUT/obj/BOOTANIMATION/bootanimation/part$part_cnt
 done
-tar xfp "vendor/cm/bootanimation/bootanimation.tar" --to-command="convert - -resize '$RESOLUTION' \"png8:$OUT/bootanimation/\$TAR_FILENAME\""
+tar xfp "vendor/cm/bootanimation/bootanimation.tar" --to-command="convert - -resize '$RESOLUTION' -colors 250 \"png8:$OUT/bootanimation/\$TAR_FILENAME\""
 
 # Create desc.txt
 echo "$IMAGESCALEWIDTH $IMAGESCALEHEIGHT" 60 > "$OUT/bootanimation/desc.txt"