Desks: Activate default desk in desktop first behind a flag

Moves the activation of the default desk in desktop-first displays
behind a new flag, as it causes multi-display CTS failures.

Flag: com.android.window.flags.enable_multiple_desktops_default_activation_in_desktop_first_displays
Bug: 408521020
Test: atest DesktopDisplayEventHandlerTest
Test: with multiple desks enabled, no multi-display CTS failures with
activation flag disabled

Change-Id: Ied4ca460fbcb7abe460f0b4d7203940fb5cd3635