Filter out empty desks from persistence.

Empty desks with no tasks in them do not need to be persistent. So when
updating persistent repo, filter them out.

Test: atest DesktopPersistentRepositoryTest
Fix: 416037263
Flag: com.android.window.flags.enable_desktop_windowing_persistence
Change-Id: Ie64b810acc2c423deeafd92e4b61a92ee6903ee6