Fix pip dismiss not visible on phone landscape

The increase to the gradient height doesn't work well for phone
landscape since it can be larger. This meant the calculation to
place the dismiss target window is incorrect.

This change fixes it by capping the height to the window size.

Test: visual - have PIP in landscape on phone and verify drag to
               dismiss target is fully visible
Bug: 223467425
Change-Id: I78a5565c8e09f010fad46ccd96c137f1c4de93b0