Verify the exact display count for display window listener test

Even in unit test, RootWindowContainer#setWindowManager still adds
the displays from the real display manager. If the testing device
has more than 1 display, the test will fail.

Bug: 169809706
Test: Enable simulated display and run
      atest ActivityTaskManagerServiceTests#testDisplayWindowListener
Change-Id: I3aa53d3a0f1907083b748abfc2fb2803040c95dc