overview: Continuously fade no recent items label

During continuous trackpad swipe to enter/exit overview, the no
recents label should have an opacity value between 0 and 1 depending
on the scroll distance.

Some variables are piped through overview grid to ensure we can
create the label with 0 opacity and no animation.

Test: ash_unittests *ContinuousOverviewAnimationTest.NoRecentItemsLabel*
Fixed: b/295063869
Change-Id: I76a79645cf53be948f389cd75dfce58f23da9493
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4823151
Reviewed-by: Connie Xu <conniekxu@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1190886}