Only automatically enter hub when exiting dream

The UX specs call for the lockscreen to never show when the hub is
enabled and the device is docked. While this is implemented, we don't
have a custom animation for it and it's new behavior that users may not
expect before we announce it. For now, only automatically switch to hub
when exiting the dream

Bug: 322787129
Bug: 327414957
Test: atest CommunalSceneStartableTest
Flag: ACONFIG com.android.systemui.communal_hub STAGING
Change-Id: I3b97b6f7b43fe504edabaf3e588fb93868970d49