Add support for back action in split shade

Split shade, unlike single shade, supports the back action to
collapse it. Many tests rely on this functionality, so this should
fix a bunch of large screen E2E tests that fail with Flexiglass
enabled.

Bug: 394840183
Test: Ran failing large screen shade tests
Test: Manually verified back action works
Flag: com.android.systemui.scene_container
Change-Id: I995a457e942b6aa66362bc89f5f713d344359ffd