Replace usages of CompletableFuture

[ryantrem]

The Android implementation currently uses CompletableFuture, but this is not available in any API level below 24, which is higher than the minSdkVersion that we want to support. As such, we need to remove usages of CompletableFuture and replace them with some alternative.

[ryantrem]

See https://developer.android.com/about/dashboards/ to determine whether we should support less than 24.