Use PNG compression level 6 for Android resources

[PrestaEdit]

Follow-up to #399. When android.image_format is png (the default), launcher icons and splash screens are written with imagepng(..., 9). Level 9 is very slow for a tiny size gain over zlib's default level 6. PNG compression is lossless, so the output pixels are identical either way.

Measured on a 1280×1920 xxxhdpi splash:

Level	Size	Time
0 (before #399)	7.04 MB	0.03 s
6 (this PR)	1.40 MB	0.53 s
9 (current)	1.28 MB	9.35 s

With up to 10 splash variants (5 densities × light/dark) plus icons, level 9 adds roughly a minute to every Android build for ~2% smaller files. The WebP path is unchanged.

Same level as the 3.x backport in #534. tests/Unit/Concerns passes.

🤖 Generated with Claude Code

[shanerbaner82]

Thanks for the numbers, that's a big build-time win. One small thing: the code comment right above the call in resizeImage() still says // Level 9 = max PNG compression.... Could you update it to match level 6 (e.g. note it's zlib's default and why 9 isn't worth the build time)?

[PrestaEdit]

Done, updated the comment. Thanks!