Refactoring of WMS.addAppWindowToListLocked.

1) getTokenWindowsOnDisplay can be private;
2) in addAppWindowToListLocked the first operation should be checking
display content;
3) adding window to exiting window token extracted to a separate
method;
4) fetching window list directly from the display content, instead of
going through the window.
Change-Id: Ibe196fb6eea15b7800938bdaefcbe8cd1a76ca14