Add bottom sheet describing lock screen widgets

When user opens edit mode, we will show a bottom sheet containing some
text around lock screen widgets. The user will only see this
once, and will not be shown again after it is dismissed.

This change also refactors the CommunalPrefsRepository to be more
generic, and introduces new CommunalPrefsInteractor for the business
logic.

Fixes: 341375291
Test: atest CommunalEditModeViewModelTest
Test: atest CommunalPrefsInteractorTest
Test: atest CommunalPrefsRepositoryImplTest
Test: atest CommunalInteractorTest
Flag: com.android.systemui.communal_hub
Change-Id: Idc618797bf7691654acd028d692d049c32c99218