Allow invisible private presentations

Private presentations can be shown only on private virtual
displays, so they are exempted from our new presentation policies.

However, one of the new policies introduced with
I33471146e072c6c53ef2ce8e1404394fb17f140a was unintentionally
applied to private presentations, thus it's causing a regression
on some systems that rely on private presentations to show UI.

Flag: com.android.window.flags.enable_presentation_for_connected_displays
Bug: 420923508
Bug: 417406465
Test: PresentationControllerTests
Change-Id: I7bbe7677059b6ece6c8f713c9e622b4e067f0ed7