Provide single threaded executor to UnfoldUnfoldTransitionFactory

This fixes a concurrency issue where HingeSensorAngleProvider was being stopped and started at the same time in a thread-pool after a fast fold/unfold, despite not providing concurrency guarantees.

In sysui, the background executor provided was already single threaded, so no issue arisen. From Launcher, THREAD_POOL_EXECUTOR was provided.

In a follow up cl, I'll add a @SingleThreadBackground annotation to the executor used in the unfold lib.

Bug: 261320823
Test: manually stress tested fold/unfold.
Change-Id: Iccf1f1f7246d8592d4d80a032479aa75f0050655