[Flexiglass] Add horizontal paddings to the SingleShade

Add horizontal paddings to the SingleShade, to match the looks of the
legacy design. It only affects wide screens.

Fixes: 364198236
Bug: 360064867
Test: open the shade in different orientations, and compare it with the
legacy behaviour side-by-side
Flag: com.android.systemui.scene_container

Change-Id: I96a5a4084efa4eebef1626b3f7371ffde46ff906