Avoid request threads on AppEngine Java 8

While the code had correctly determined full threads were available, the
call to MoreExecutors returned a request thread factory, which has
limitations.

Note that Async stub users may not be able to call GAE APIs in
callbacks. This is because the threads aren't request threads. They can
override the individual call's executor with
com.google.appengine.api.ThreadManager.currentRequestThreadFactory() in
an interceptor via callOptions.withExecutor().

Fixes #3296