Introduce DisplayGroup

A DisplayGroup is a collection of LogicalDisplays which are tied
together for certain behaviors and operations. This CL simply introduces
the class, however in the future it will be used for multi-display power
state work.

Bug: 138328918
Test: manual - build and deploy
Change-Id: I3d736b2758a3363678afc962872fcbe7f4e96806
8 files changed