Add CompletableFuture based API option #4
Description
We should support Java8' CompletableFuture. In Android it's only available on API level 24+. It will enable us to provide an alternate to the current callbacks approach.
NB: An issue for providing CompletableFuture for older versions of Android exists with upstream https://issuetracker.google.com/issues/153794291. With Android Studio 4.0 the new desugar support was improved and we may eventually see the upstream embrace it.