Replaces manual offloading of some method implementations to a
background task queue with just using the new method channel task queue
system to have those methods be called on a background queue
automatically.

This simplifies the code, and also allows removing the Guava dependency with
a couple of other minor changes:
- Replaces `Joiner` with Java 8's `String.join`.
- Replaces `Strings.isEmptyOrNull` with a local helper method.

Fixes https://github.com/flutter/flutter/issues/13991