[flexiglass] Remove NotificationsShadeSceneContentViewModel.

ag/28627066 split the notifications shade scene view model in two: one
for actions and another for content. The latter, however, doesn't hold
any state that's used in notifications shade. The base class exposed
a shared `onEmptySpaceClicked()` handler, which shouldn't be used in
dual shade (`OverlayShade` handles clicks on the scrim), so this CL
merges it directly into `ShadeSceneContentViewModel` instead.

See the discussion in ag/28627066 for more context.

Bug: 340553529
Flag: com.android.systemui.dual_shade
Flag: com.android.systemui.scene_container
Test: Manually verified by opening and closing the notifications shade
in dual shade, and regular shade in Flexiglass.

Change-Id: I63371c63d380bfe2e1f37d92ebc06d69b08524de