[docs] Update app-icons.md (#9313)
[docs] Update app-icons.md (#9313)

* Update app-icons.md

Included the `app.json` key for setting the foreground image when using android adaptive icons

* Update app-icons.md

Co-authored-by: Charlie Cruzan <2953b215c2408c6fcd3f838b7ad96c528f0f09e6@users.noreply.github.com>
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