Update the app window's thumbnail layer after starting window is removed

This fixes the case when bottom app is resuming in portrait mode on a
tablet where the thumbnail's title bar appears over the docked app (from
the top) only when device slows down (or slow down the animation). This
is caused by the StartingWindow removing itself from the display and not
updating the thumbnail that is still animating while all the other
windows updated their z-ordering. When the apps update their z-order,
they lower the dock stack's z-order because the starting window (which
was behind the dock stack) is gone. However the thumbnail still has the
same z-order meaning that the thumbnail will sit on top of the docked
app. So when the starting window disappears, the z-order for the thumbnail
will also update to fit behind the docked app. Also simplified the
thumbnail layer code.

Fixes: 35860227
Bug: 62029108
Test: manual, dock an app in top and bottom, go to recents, slow down
device animations by 5-10x, resume any bottom task
Change-Id: I79bc92b79e50a7b646b7b6c22802e55e04cc1799