Default ThreadFactoryBuilder backing factory to platformThreadFactory()

[cpovirk]

Seems like probably the right move, since we've moved to use platformThreadFactory() elsewhere when creating threads (for App Engine reasons).

[cpovirk]

Or there may be no need, as platformThreadFactory() may no longer require special treatment for App Engine: #3598

Alternatively: All the more reason to make the change when it would be a no-op, preparing us for any future day when it might not be :)

[cpovirk]

I tested this internally, and it broke something. I didn't investigate further.