Debounce HeadsUpNotificationInteractor.isHeadsUpOrAnimatingAway

When the last HUN is unpinned, and we are starting the animation, we
should keep this flow true the entire time. This prevents flickering.

Bug: 342495652
Bug: 347489544
Test: atest HeadsUpNotificationInteractorTest
Test: systemui-notification-3-jank-suite
Flag: com.android.systemui.notifications_heads_up_refactor

Change-Id: Id9c8c22fbc0da6cec4469fc5ce74cc58ce16dd90