Allow Task#setDragResizing on W_M_MULTI_WINDOW

This CL unlocks the capability to set Task#setDragResizing for tasks in
WINDOWING_MODE_MULTI_WINDOW.
With Task#setDragResizing, some resizing optimizations (e.g., using a
fullscreen-sized surface to reduce reallocations) are applied while
resizing.
Because W_M_MULTI_WINDOW is also allowed to be resized from the Shell,
we need such optimizations for W_M_MULTI_WINDOW as well.
It used to be allowed for legacy docked tasks, but the docked mode
refactoring accidentally cleaned this up. This CL restores the
capability.

Bug: 308722494
Test: TaskTests
Change-Id: I4dc9150ebbcc0cb55eddb7d52090da6e983fe06e