Share selection state between BaseCommunalViewModel implementations.

CommunalViewModel and the edit variant both separately store selection
information. This leads to ferrying of selection data between the
two models which can get out of sync. This changelist addresses it by
having all BaseCommunalViewModel subclasses share a single selection
flow.

Test: atest CommunalViewModelTest#selectedKey_changeAffectsAllInstances
Test: atest CommunalInteractorTest#settingSelectedKey_flowUpdated
Fixes: 359419762
Flag: EXEMPT bugfix
Change-Id: I0bab6f29016f61f9bc2413f5ac22f5e6a65477ac