[Flexiglass] Extract QS layout to use slots

This simplifies the portrait vs landscape layouts.

Test: manual, expand QS
Test: atest android.platform.test.scenario.sysui.quicksettings
Test: atest android.platform.test.scenario.sysui.media
Bug: 420960164
Flag: com.android.systemui.scene_container
Change-Id: I3228c6e28bd723fab35fc252f44fe36fa03e3231