Add DisplayModeDirector to determine set of allowed modes.

SurfaceFlinger now knows how to automatically schedule between various
display modes, so we need to tell it what modes are available to switch
between based on overall system state as well as display specific
properties. To capture all of this system state we've introduced
DisplayModeDirector which monitors all of the various inputs for
deciding display mode and notifies the rest of the display
infrastructure when they change.

Bug: 123727652
Test: manual
Change-Id: I83184664bf63c99ebd31889764720bb55c2e15a8