Desks: Do not separate recent tasks into desks without frontend

Check both flags before grouping tasks into per-desk GroupedTaskInfos,
otherwise if there are multiple desks (which can happend during
debugging or by disconnecting an external display) Launcher might try to
create separate Recents tiles for each desk.

Flag: com.android.window.flags.enable_multiple_desktops_backend
Flag: com.android.window.flags.enable_multiple_desktops_frontend
Bug: 414498479
Test: create additional desk with adb command - verify there is only one
desktop tile in overview

Change-Id: I8d4a1e141b7117fbb56de10cb4f811bdd08e843d