Handle visibility of AddDesktopButton for overview/quickswitch

This change introduces mDisallowScrollToAddDesk to handle the visibility
of AddDesktopButton in overview/quickswitch. In overview, set its scroll
as the first task; in quick switch, set its scroll out range of
[minScroll, maxScroll].

Bug: 398036110
Flag: com.android.window.flags.enable_multiple_desktops_frontend
Flag: com.android.window.flags.enable_multiple_desktops_backend
Test: manual quick switch
Change-Id: I5fada0c4c5bccc5572b458da9970e3524087508d