Scale PiP to stack instead of crop.

When changing stacks the finalCrop or crop may not be set
so relying on either one for the scaling factor is error prone.
Just scale to the stack size. I initially avoided this because I
thought some of the crop code may be relevant in computing the size
we wan't...but I'm now pretty sure it won't.

Test: Expand PiP back to fullscreen on phone, verify no crash.
Change-Id: I172daade74bec7374b5fd3310c1bf554e46d8832