CommunalStateController Introduction.

This changelist adds the CommunalStateController, which
is responsible for reporting the whether the communal
view is showing and other state. The communal view state
is reported through the CommunalSurfaceViewController when
shown and CommunalSurfaceHostViewController when hiding.

Bug: 195601027
Test: atest CommunalSurfaceViewControllerTest#testCommunalStateControllerShowNotified
Test: atest CommunalHostViewControllerTest#testCommunalStateControllerHideNotified
Test: atest CommunalStateControllerTest
Change-Id: I2a18a50c8c78e811cd234773994f4591a32303b5