Clarify order of appinstalled event and prompt() result

[beaufortfrancois]

Spec is not clear on whether the appinstalled event should fire before or after prompt() promise resolves. This may lead to compatibility issues if not addressed.

https://wicg.github.io/manifest-incubations/index.html#dfn-present-an-install-prompt
https://wicg.github.io/manifest-incubations/index.html#dfn-steps-to-install-the-web-application

I would expect the events to fire in order, user responds to the prompt, then the install to happen.

1. beforeinstallprompt event fires
2. prompt() resolves
3. appinstalled event fires

Chromium issue: https://bugs.chromium.org/p/chromium/issues/detail?id=1094025