Wait for transitions before starting a test scenario

This CL lets the tests wait for any transitions to be finished before
running each scenario.
There might be an unfinished transition from the previous test’s
teardown process.

Flag: EXEMPT - test stabilization
Bug: 442453305
Bug: 442453913
Bug: 442454448
Bug: 441594940
Bug: 442453123
Bug: 442453799
Test: atest WMShellSmokeTestsDesktopMode
Change-Id: I825b22abf6be5d743c9e2de9c44d1ab223a7f15f