Drop initial default values from flows.

Bug: 387511656
Flag: com.android.launcher3.enable_refactor_task_thumbnail
Test: Checking perfetto traces shows that this reduces the number of continuations on main by ~100 for 30 apps
Test: Opening Overview shows correct content.
Test: TaskThumbnailViewScreenshotTest
Change-Id: If8d3c6a8f82cea38a3fb78339946bec2c8646ab7