[flexiglass] Set the horizontal shelf alignment based on the ShadeMode

With the minimal HUN feature we'd like to display a "short" shelf only,
without any notifications. This "short" shelf would look weird on the
middle of the screen, so we'd prefer to align it to the closer edge.
Currently this feature is not supported well on DualShade, and this CL
fixes that.

Fixes: 394312485
Test: check that the shelf is aligned to the closer edge of the screen in all shade modes
Flag: com.android.systemui.scene_container

Change-Id: Ia048bd56a9465708a2cd8f2a7f03f85b2fa48e40