are android animations always within the UI Thread?

Your thread has no real effect here. The call to mView.post() essentially schedules that runnable to execute on the main thread in its next available cycle. You might as well save the effort of starting a new thread and do it all on the main thread. There is not much going on here.

Animations work by scheduling lots of tiny bits of work to run on the main thread with each frame, which is about every 16ms. This is very typical. Unless you are seeing a very specific problem that you need to optimize, there is no reason to avoid doing this kind of work on the main thread.