Make overlay run in background instead of unconfined (main)

This means that when TaskOverlayViewModel.overlayState is combining flows it does it on background. If this combination results in an emission then there is still a message to main but if it does not emit (e.g. no change filtered by distinctUntilChanged()) then it will not result in a continuation on main.

Bug: 387511656
Flag: com.android.launcher3.enable_refactor_task_thumbnail
Test: Checking perfetto traces shows that this reduces the number of continuations on main significantly
Test: Manual - overlay presence + select functionality
Test: Presubmit (TAPL/image tests etc)
Change-Id: I840cb86e91db5a0626191b5eab145978ef4192e9