Wait for taskbar after connecting an external display

This CL lets cuj1e/p to explicitly wait until the taskbar is available
after connecting an external display.
Otherwise, on slow devices (e.g., cuttlefish) the settings app launches
before the external display is fully configured.

Flag: EXEMPT - fix test
Bug: 441687297
Test: ConnectedDisplayCujSmokeTests
Change-Id: I1225038727d216f4299b734d08bd88fd145c27a2