[Flexiglass] Enable closing the shade with back button

Add Back action to shade actions view model.
This enables closing the shade with back button
as in non-flexiglass implementation.

Fixes: 430910634
Test: Back global actions closes the shade
Test: `adb shell input keyevent 4` closes the shade
Flag: com.android.systemui.scene_container
Change-Id: I4d33f66f092e66d943e369ed1d781b990a9e6f06