Update screenshot UI elevations

Screenshot preview used to be atop the actions container, so it had
higher elevation. Now it's separated, so moving the relevant views'
elevations down to match the actions.

Bug: 329659738
Test: Visual inspection with a discerning eye
Flag: ACONFIG com.android.systemui.screenshot_shelf_ui DEVELOPMENT
Change-Id: I6dc7755c7f3e84052caa8d9b5abc323976650d77