[Dual Shade] Expand the other shade when swiping down the opposite half.

Fix: 417947092
Test: Tested manually by swiping down on the empty side when either
 shade was open (on a wide-screen device), and verifying the expected
 behavior.
Test: Updated unit tests.
Flag: com.android.systemui.scene_container
Change-Id: Iefa588ee13888ce156e1f6c6924b997445923333