[flexiglass] Fix NSSL eating touches on empty space in decor views and margins

NSSL should now only run touch handling if a gesture started on an actual child view

Bug: 296118689
Test: manual
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Change-Id: Iadf72a0f818b07563bda6060012cf87cc6501f59
2 files changed