Fix the bevy templates

[mirsella]

Describe the bug the bevy-demo template doesn't even compile anymore, use 2018 version, and bevy version 0.3.0.

Steps To Reproduce cargo mobile init with bevy template

Expected behavior up-to-date working template

[jmetz]

I've cobbled together a stripped down version of @niklasei 's bevy_game_template, modified to use a working cargo-mobile2 template here: https://github.com/jmetz/cargo-mobile2-bevy-example

Perhaps this could help develop the templates.

[FabianLars]

https://github.com/tauri-apps/cargo-mobile2/pull/463