Move EmptyShadeViewModel flows to the bg

Bug: 371134540
Bug: 366003631
Test: v2/android-crystalball-eng/health/microbench/systemui/main/systemui-quicksettings-2-jank-suite
Flag: android.app.modes_ui_empty_shade
Flag: android.app.modes_ui
Change-Id: Ida1966cdfa70683dd866edd83b6d2631c1cfb4b8