Switch elevation from dp to px in shadow test

Using px and dp in the same layout file causes the test to fail on
xxxhdpi screens in 3840x2160 mode.

Test: atest 'android.uirendering.cts.testclasses.ShadowTests#testShadowLayout'
Bug: 200681610
Change-Id: I8f40c581c4a611a4704ef843a8d3a59fb4ffe6ad