Proposal: Add await for play-services integration for Task constructed with CancellationToken

[alexvanyo]

The current play-services integration provides Task.await that allows waiting for the result of an already created Task. This function supports cancellation, but not in both directions: If await is cancelled, then the underlying Task is not cancelled.

Some methods of creating Tasks don't support cancellation in this manner, but some do by creating the task with a CancellationToken. For an example: https://developers.google.com/places/android-sdk/reference/com/google/android/libraries/places/api/net/FindAutocompletePredictionsRequest.Builder#setCancellationToken(com.google.android.gms.tasks.CancellationToken)

My proposal is to add another await method, that operates on a lambda that constructs a Task given a CancellationToken.