Disallow a new presentation to show on presenting display

canPresent() has a check to prevent multiple presentations on a
single display but this isn't working for a new presentation
because of the null check of |win| (|win| == null means a new
presentation as the window hasn't been created).

Removing this null check allows the existing logic to work for
new presentations too.

Flag: com.android.window.flags.enable_presentation_for_connected_displays
Bug: 418850221
Test: PresentationTest
Change-Id: I29ccb089aa057616138d039b3d62697a5ed63892