Disallow PiP window size smaller than the spec

On a foldable where the aspect ratio of inner display is close to
square, it's possible a minimal size on inner display become too small
when transit to folded mode.

Reset the new bounds to minimum allowed size in
PipController#onDisplayChange when that does happen.

The newly introduced method in PipController is annotated as
VisibleForTesting, unit test for that class is underway and tracked
separately in b/394941160.

Flag: EXEMPT bugfix
Bug: 433610267
Video: http://recall/-/aaaaaabFQoRHlzixHdtY/dLES4FueYLSboIcUZINxii
Test: Resize PiP to minimum in unfolded state, then fold, see Video
Change-Id: I021724a11c2df7410af987f651866deae54c0861