Relayer taskbar to application_overlay when shade is opened

When we get the sysui flag for shade expanding, take a
snapshot of the TaskbarDragLayer (after hiding three navigation
buttons), and add a View of that snapshot to the TaskbarOverlay
layer, while simultaneously hiding the real Taskbar.

Bug: 343194358
Test: have pinned taskbar, open shade, note taskbar stays behind shade
Flag: com.android.launcher3.enable_taskbar_behind_shade
Change-Id: I0ad9506f34dd4f3aec3796d94cd3d4a914ff67e5