Implement veiled resizing.

Implements a veil that covers app contents during drag resizing.
Video: http://recall/-/hJNEr4C0IUowK3TyPPf4wT/c8b2u7opjuEJh7NrNhT3hS

Bug: 274773589
Test: Manual; drag resize tasks in desktop mode and confirm resize is
veiled.
Test: atest TaskPositionerTest

Change-Id: Iba789762283a3359a78476d557075c1bc427a3d5