Flexiglass: Fix clock click callback on desktop

On desktop devices, it does not make sense to launch the clock app when
clicking on the clock in the shade header. We should show/hide the shade
instead.

Bug: 408486241
Flag: com.android.systemui.scene_container
Test: ShadeHeaderViewModelTest
Change-Id: I4fdfc2e35ba693126c9a1bf60b22bad02b5155c5