Optimize window relayouts when dragging divider

- Communicate the resize mode to ViewRootImpl. If it is a docked
divider resize, do not call relayout for every traversal.
- Do not call Task.resizeWindows() unconditionally when changing
Stack bounds. It might be just a move.
- However, not calling relayout breaks layout bounds while
relaunching. To fix that, do the following:
-- Inform ViewRootImpl that the activity just relaunched, and force
a relayout call in the next traversal so the client can pick up
the unfrozen bounds.
-- When unfreezing the bounds, cause a traversal in window manager.

Bug: 25015474
Change-Id: Iab9a445dabce4f6ec1e25957038fe40a4be65246