define the behavior for consecutive |getAvailability| #335

Three unclear behavior found while implementing the |getAvailability| algorithm:

1. In step 6, the same availability object is returned if an existed (A, presentationUrls) tuple is found. However, we can only return the same object if the requester is in the same DOM window.
2. in step 9, we should run the device monitoring algorithm in parallel so that we can resolve the promise as early as possible.
3. For consecutive |getAvailability| on the same PresentationRequest object, we should guarantee the resolve order in the algorithm. For example,

request.getAvailability().then(function() {console.log(1)});
request.getAvailability().then(function() {console.log(2)});

should always print "12" in console for all browsers.