Currently, if we create 4 (upper limit) non-empty desks, reboot, and
enter Overview, the addDesktopButton shows up when it actully shouldn't.
This change fixes this edge case by checking if we can create a desk or
not when inflating addDesktopButton and update its visibility
accordingly.
Bug: 416052493
Flag: com.android.window.flags.enable_multiple_desktops_frontend
Flag: com.android.window.flags.enable_multiple_desktops_backend
Test: create desks and reboot
Change-Id: I7142c3aeb88cc1f06b99ddef753ea20c63b17297
Suhua Lei
a6bd2abc12