When keyguard exits use same anim for all windows

The entering animations were only applied to the incoming windows
one time. If those windows weren't drawn yet then they never had
an animation assigned.

Furthermore if a starting window was drawn in time it would get the
animation but its main window would not get it if it weren't drawn.
Even if an animation were assigned later they wouldn't be synced
with each other.

This change creates a single animation which is shared by all
incoming windows. As windows are drawn they can then animate with
the starting window.

(Also refactorings to eliminate redundant code and unnecessary
variables.)

Fixes bug 15991916.

Change-Id: I9949ef0a1639c831754316da34de97cb86403f5a