Don't keep visible windows in pinned stack on screen when app dies

We previously introduced logic that keeps an apps visible windows on
screen when the app dies. This was to help with the situation where
freeform apps might be killed by the low memory killer and we want
to preserve the space on screen and relaunch the app when the user
interacts with the window again.
However, this doesn't work for windows in the pinned stack since
they an normally not focusable/interactable.
We no longer do this for windows in the pinned stack.

Bug: 24913379
Bug: 26609941
Change-Id: Ie2e7f7d7c7a8c0ef6c1662517f558c385201c433