Make a distinct launch icon for the app

[gnprice]

Currently the app's icon, as seen on e.g. the device's launch screen, is the one the flutter create template gave it: just a Flutter logo.

That was OK for a prototype, but it's a little odd for an app people actually try out and use. For the beta, even Beta 1, we should give it some Zulip-specific icon.

It is useful, though, that the icon is visibly different from the one we use for the main app. That helps for telling the difference on the launch screen, and in other system UI where different apps appear.

For debug builds of the zulip-mobile RN app we have a variant icon that overlays the word "DEBUG". We can do something analogous here.

[gnprice]

Relatedly, the app's title as shown on the system launch screen is "zulip". (For the main app, it's "Zulip".)

It should instead be something properly capitalized, and more specific: something like "Zulip (beta)" or "Zulip (Flutter)".

I'll just make that change at the same time as the icon.