Scale light z value relative to screen size

On larger screens, having the light z value lower down creates an
undesirable amount of shadow since the source to view angle can be quite
large. By scaling the z position of the source relative to the screen
size, we can avoid these extreme shadows.

Bug: 227425789
Test: run cts -t android.uirendering.cts.testclasses.ShadowTests --module CtsUiRenderingTestCases
Change-Id: I8b7a3fcf93daaa565256a08f38deffed74b24ad4