wm tracing: fix race condition in tests

The perfetto ds registration is an asynchronous operation, hence
WindowTracingPerfettoTest should busy wait till the ds is actually
registered/available before starting any tracing session.

Flag: EXEMPT bugfix
Test: atest WmTests:WindowTracingPerfettoTest
Bug: 323165543
Fix: 360290192

Change-Id: Ib750b381343c12922927a0a81668d0935327c990