Refactorings for Window Manager module.

Following improvements were applied:
* extract code from a very large method
WindowSurfacePlacer.performSurfacePlacementInner into
WindowsurfacePlacer.applySurfaceChangesTransaction; smaller methods
are easier to understand;
* WindowStateAnimator.showSurfaceRobustlyLocked can be privatized, it
is only called from one place; also there is unnecessary check for
whether mSurfaceControl is not null;
* prepareAppTransition code can be mostly moved into AppTransition;
it calls mostly methods from this class; as a result some methods
from AppTransition can be privatized;
* requestTraversalLocked can be moved into WindowSurfacePlacer, which
allows mTraversalScheduled to be a private field inside the placer;
this way WindowSurfacePlacer can nicely control and hide the need for
layouts.

Change-Id: I99006c859ef224f5dad933f5c15d2cb2b9738a9e