Desks: Obtain default desk if needed behind flag

Moves usages of #getOrCreateDefaultDeskIdSuspending behind a flag.

Flag: com.android.window.flags.default_desk_without_warmup_migration
Fix: 410595598
Bug: 406890311
Test: move to desktop using handle menu, no coroutines invoked
Change-Id: I76d416e76dfa22101a5c9e1f71fbaea19d0bf50c