CommunalService Implementation.

This changelist introduces CommunalService and
related interfaces. These interfaces allow other
components to specify a communal surface to be
displayed in SystemUI.

This changelist also includes an implementation
of CommunalService that wires up the previously
defined communal support in SystemUI.

Test: manual
Bug: 193911222
Change-Id: Idf38777f948d877ad299c75fa76ad396fa035e06