Fix taskbar linger above notification shade

Remarks:
1. I think the correct fix for this would be to have the Taskbar z-ordered below the notification shade. That however seems to be difficult because there are cases when the taskbar window must be above the notification shade.
2. This CL improves the behaviour by starting the taskbar disappear animation when the notification panel is half expanded instead of waiting for the full expansion. This improves the UX when expanding the shade slowly. When expanding the shade quickly, this CL does unfortunately not significantly improve the UX.
3. I believe that the `SYSUI_STATE_NOTIFICATION_PANEL_EXPANDED` and `SYSUI_STATE_QUICK_SETTINGS_EXPANDED` flags can be replaced by the newly introduced one. But since this would pose the risk of introducing new bugs, I did not do that in this CL. It is my intention to create a CL with that replacement in udc.

Bug: 272621219
Test: Manual, i.e. observe Taskbar behaviour when pulling down notification shade and expanding quick settings
Change-Id: I10eb4d211f614559167f110000dcec3c8df5e3c8