SF: Store display modes in DisplayDevice

Make DisplayDevice the source of truth for display modes
and call HWComposer only on hotplug in order to populate
the state of DisplayDevice. Additionally rename s/config/mode
in DisplayDevice.

Bug: 159590486
Bug: 175678215
Test: atest libsurfaceflinger_unittest
Change-Id: I3d8bcbf8f74d910213e0cbe3bcdd839323076e56