[DesktopStatusBar] Hide lockscreen status bar

The DesktopStatusBar is visible irrespective of lockscreen
state. Therefore, we can hide the KeyguardStatusBar in a
similar fashion to the ShadeHeader.

Test: Manual
Test: KeyguardStatusBarViewModelTest
Bug: 438499562
Flag: com.android.systemui.status_bar_for_desktop
Change-Id: I6206f6395131058a8e58091d6df0a5fbdd121e49