Desks: Use a single transition to remove all desks

Instead of starting one for each desk, which can lead to races when
removing all desks using the "clear all" button that result in more than
1 default desk getting created.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Fix: 427265290
Test: remove all desks with adb command or overview's "clear all"
button. Check all desks are removed and there is at most 1 newly created
desk (in desktop first, 0 desks in touch first).

Change-Id: I591186923cf33aa19040522f955c8ecf2d8ac8c8