Remove canShowAsBubbleBar and instead use isShowingAsBubbleBar

Typically we want the 'isShowing' check instead of 'canShowAs' so remove
that to avoid confusion.

Flag: com.android.wm.shell.enable_bubble_bar
Test: atest WMShellUnitTests BubbleBarLayerViewTest
Bug: 392963712
Change-Id: I63158785cd1c7ad3e9eca8534c4fa5a22a7b496b