Add test mechanism to check whether the shell drop target window has drawn

- In rare cases on CF, the time to draw the shell drop target can be
  longer than the entire tapl drag gesture.  Instead, we expose a
  call that Launcher can use to check whether the drop target window
  has drawn since it was last made visible which tapl can wait for
  when invoking a drag.

Bug: 234653212
Test: atest NexusLauncherTests
Test: atest DragAndDropControllerTest

Change-Id: Id152b05651f1646a12e9e7b4bb1b7e52c4a9ecea