Add CI for android

[Zoxc]

This just runs cargo check on eframe for an Android target. It doesn't currently pass due to arboard intentionally not building on unsupported platforms. Not quite sure where that should be resolved.

[emilk]

Let's add [target.'cfg(not(target_os = "android"))'.dependencies] before the arboard dependency in egui-winit/Cargo.toml

[Zoxc]

I did the arboard fix and added some fixes to eframe too.

[emilk]

This is great; thank you!