[Dual Shade] Limit the shade panel width to half the screen width.

This only applies to the shade on wide screens.

Fix: 391379390
Bug: 338033836
Test: Manually tested by opening the shade on a folded device, an
 unfolded device, and a tablet and observing the behavior matches the
 UX spec.
Flag: com.android.systemui.scene_container
Change-Id: Idd815d3e8ed0f6336cbeb863bdbadc1a814f1220