Add WCT#setDragResizing to optimize fluid resizing

Allows WM Shell to indicate the start/end of drag resizing, which
core can use as a signal to reuse a single (larger) surface size
for the entire drag resize operation to avoid continuous buffer
allocations after each size change.

Bug: 249808500
Test: drag resize a freeform window, verify WindowLayout requests
a fullscreen sized surface; atest TaskPositionerTest

Change-Id: I27e2b44270d7ea4f701fa8037f93b20dc691284b