More optimization while dragging docked divider

- Make sure mPendingBackdropFrame gets also set when if the window
triggers a relayout on it's own, so it doesn't call into window manager
all the time.
- Set the insets of the docked divider to empty so we don't trigger a
layout when we are just moving it - it doesn't need it in any case.
- Send a window move message to the divider when it moved
- Update attach info in all move cases, update light center

The whole resize operation now only takes around 4ms per frame, and
leaves a lot more resources for the apps to do configuration changes.

Bug: 25015474
Change-Id: Ica48129570a0fc858a89c21f46abf3442efb0224