Set default desk limit to 4.

This limits the number of desks available in desktop windowing mode to
4, instead of being unlimited.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Bug: 362720497
Test: open 4 desks on any device, verify Overview cannot create any
new ones

Change-Id: I2f5d50d19670c74064f9437efac22284fc6ac3d3