Disable elevation shadows when box shadows are enabled

Bug: 367464660
Bug: 421025291
Test: Enable com.android.wm.shell.enable_pip_box_shadows and run the following test. Take a perfetto trace during the test and ensure that elevation (drawShadow) and box shadows (BoxShadows) aren't rendered simultaneously.  atest systemui-desktopmode-drag-1-jank-suite    --request-upload-result    -- --enable-module-dynamic-download    --module-arg systemui-desktopmode-drag-1-jank-suite:strict-include-metric-filter:'perfetto_ft_systemui-missed_sf_frames-mean'    --test-arg com.android.tradefed.testtype.AndroidJUnitTest:class:android.platform.test.scenario.sysui.desktopmode.DragAppWindowMultiWindowAndPipMicrobenchmark
Flag: EXEMPT bugfix
Change-Id: I01193e3819160f379e267dbc9a86e4b002387e44