Reuse coroutine trace context element

In repeatWhenAttached, create a single coroutine trace element during
initialization, and re-use it so we allocate less objects.

Flag: ACONFIG com.android.systemui.coroutine_tracing DEVELOPMENT
Bug: 289353932
Test: SystemUITests
Change-Id: Ifb80bfcab663f35df92511fbbe94bef65e1982fe