Avoid request threads on AppEngine Java 8

[ejona86]

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

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@neozwu, can you test this?

As a user, I think I'd expect the Async API callbacks to be done or request threads. But this isn't very feasible to configure automatically because the executor would need to "save" the environment when a new call is created and re-associate callbacks with it. It'd be easy to override blindly, but then users couldn't override the executor.

[ejona86]

CC @garrettjonesgoogle

[neozwu]

@ejona86 with this change, the NPE is fixed. However, I got an io.grpc.StatusRuntimeException: UNAUTHENTICATED error. It seems this is due to the fact refressAccessToken is now called in a "normal" thread, and, as you mentioned, refreshAccessToken internally calls appengine API, so it will fail in "normal" thread. I will go ahead and try fix the auth lib to use metadata server instead of appengine API for authentication.

[ejona86]

@neozwu, thanks. We'll go ahead and merge this and backport it to the upcoming release.