Wait for input windows to be set before proceeding with inject for tests

Added a new test method to allow the client to wait until input windows
have been propagated to InputFlinger before proceeding to inject input.
This ensures that the inject is called when all windows have been set
up.

Bug: 123041491
Test: ListPopupWindowTest, PopupMenuTest
Change-Id: I42abfa4ea23a14ae1a003c6ee8976cc466f1d5af