Re: [w3c/manifest] Add installation prompt control flow (#417)

The thing that still bothers me about the overall design is that `.onappinstall` essentially equates to `ev.userChoice = "accepted"` - hence user choice is not really needed. I'm a little uncomfortable that we are revealing actual background processing details in the current design. For instance:

1. how long the UA takes to present the install prompt: time between prompt() called and prompt() promise  resolved - and system specific errors in the form of the rejection (which are probably non of the app's business that I can think of).
1. how long it takes for a user to make a choice: time it takes for `await e.userChoice` resolving.
1. how long it takes to write to disk. Time between `ev.userChoice` resolved and `.onappinstalled` called.  

Making .prompt() synchronous and relying on .onappinstalled masks some of the above, because all the app can know is how long did it take from .prompt() to the app actually being installed - but none of the details of the process. 

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