Force windows to be scalable during pinned animation.

We resize windows at the beginning of the pinned stack
animation when animating to a larger size, and so for some
duration a resize will be pending. We need to force the window
out of SCALING_MODE_FREEZE so we can animate during this period.

Bug: 27891386
Change-Id: I5cff599ed67f2c179e938662b6f0d99bd790aaba