[CredManUi] Dismiss system bars immediately upon bottom sheet dismissal

Before the bottom sheet dismissal animatyion is applied to the system
bars so they have to travel the whole screen to fade out.Now the system
bars are immediately set to invisible upon dismissal.

Bug: 271590672
Test: manually (see bug for recordings)
Change-Id: Ib385ed5455eceabbeadb6d3c41c4382b01a9ea74