Fixed an issue where notifications could be stuck empty

We still had some legacy code which would make sure that the alpha of
dragged views would remain the same, but that isn't just unnecessary
but could lead to bugs where the view itself was stuck with alpha 0.

Test: runtest systemui
Change-Id: I36768bebcb2831aa422bafe7b18c92a4834e7745
Fixes: 80525283