Fix flaky issue

To avoid the display content of the virtual display still not be created
when setting the windowsForA11yCallback of this one, using the method,
getDisplayContentOrCreate(), to create the display content if not
existing.

Bug: 144845224
Test: atest --iterations 20
AccessibilityWindowReportingTest#
moveFocusToAnotherDisplay_movesActiveAndFocusWindow
Test: a11y CTS & unit tests

Change-Id: Id1cc8c08042011156cec9d3c4a9784fd6f90ddf5