[Dual Shade] Align the shade to the center on narrow screens.

Fix: 395069490
Test: Tested manually by swiping down both shades on a folded and
 unfolded screen, and verifying the animation is center-aligned in the
 former, and left/right aligned in the latter.
Flag: com.android.systemui.scene_container
Change-Id: I4152fefd87cb8bbadd28578c9da0921b3ab22c57