Fix notification expand-collapse crossfade issue

Since the user can drag back and forth, the fadeOut function should not return early, when the view's alpha is already 0. It should allow going away from 0 again. This check was introduced by ag/21061907.

Bug: 272263243
Test: Manual: i.e. expanding and collapsing notifications by dragging manually
Change-Id: If36327c6db4e440e0d95f51c18035a78226796a0