PresentationRequest.getAvailability() could always return a new Promise

[markafoltz]

The steps for getAvailability() have an optimization (?) to return (sometimes) a pre-existing Promise from a previous call to getAvailability().

However this isn't consistent with other specs that always return a new Promise when starting an asynchronous action on another device:

• scan() in Web NFC
• open() in Web USB
• requestPort() in Web Serial
• watchAdvertisements() in Web Bluetooth

Those are a few examples pulled by skimming other specs, there are likely more.

The Promise-caching also produces some implementation complexity because the browser must sometimes store the Promise like a property of the request, and sometimes allocate a new object and pass it directly to script.

It would be simpler to implement, and more consistent with other specs, to always return a new Promise here.

[markafoltz]

This Blink code review has some additional conversation about why this is simpler to implement (at least in Blink.) The CL was abandoned until the spec could be updated with this behavior.