Add monochrome icon for Android launchers

[ThatOneCalculator]

Currently, the Zulip app on Android has no monochrome icon, making it look very out of place on modern homescreens.

[gnprice]

Thanks for the feedback!

It looks like the feature you're using here is "themed icons". In the system settings on my Pixel 8 running Android 14, the switch to enable that feature is labeled as "Beta". Turning it on and browsing around a bit, it looks like some apps have provided these monochrome launcher icons but most apps haven't yet.

You're certainly not alone in wishing that more apps would — here's a couple of different threads of people complaining that so many apps haven't:
https://www.reddit.com/r/GooglePixel/comments/17ceeiz/themed_icons_why_is_it_so_difficult/
https://www.reddit.com/r/android_beta/comments/16chny5/wasnt_a14_supposed_to_fix_themed_icons/

Anyway, for our reference when we go to implement this feature, here's the developer docs for it (which also covers the related #1402):
https://developer.android.com/develop/ui/views/launch/icon_design_adaptive

Because this codebase is in maintenance mode, this is a feature we'd only add in the upcoming Flutter-based app. But I definitely expect we'll implement it there at some point after that migration is complete.

[gnprice]

I'll fix this in the new app as a side benefit of the work for #1254.