Fix keyguard elements showing during dream->hub transition.

There is a bug currently where if the shade is swiped down over the
dream, we reset keyguard alpha back to 1 when the shade is closed. This
is causing the keyguard to be visible when we transition away from the
dream and into the hub.

This fix only sets the alpha on the keyguard when opening/closing the
shade if the lockscreen is visible. That prevents setting the alpha when
in other states, such as dreaming or glanceable hub.

Bug: 325102385
Test: atest KeyguardRootViewModelTest
Flag: ACONFIG com.android.systemui.communal_hub TEAMFOOD
Change-Id: I9c22dde36ab02b6fbb9aaba276639d5f27128e41