CommunalSource Introduction.

This changelist introduces CommunalSource, an interface for
interacting with entities that offer communal surfaces to be
shown ontop of the system interface. It also includes support
classes, such as the CommunalSourceMonitor, for coordinating
discovery and interaction with CommunalSource.

Bug: 193745252
Test: atest SystemUITests:NotificationPanelViewTest#testCommunalSourceListening
Change-Id: I48ea711edb241d95d713deac1836a93843bbb876