Multi-window optimizations #1

- When the window doesn't have a surface, do not add it to
mResizingWindows, so we don't report unnecessary resizes
- computeDragResizing => false when window is not visible, so we
never enter resizing mode even if the window decides to relayout
in the background

Change-Id: I8e6cdef86f1579d128973d4f2f12e87bf9b65524