Read default values for dream when to start

We should consider the default config values for each setting if the
values aren't explictly set.

Bug: 383208131
Flag: com.android.systemui.glanceable_hub_v2
Test: atest CommunalSettingsRepositoryImplTest
Change-Id: Ie1c7350e49d6ea1646288600802bf7bf1606a9c5