Don't run large virtual displays through HWC

This change watches for a MAX_VIRTUAL_DISPLAY_DIMENSION value, which
will be set (if necessary) in BoardConfig.mk. If the value is set,
any virtual displays that have a width or a height greater than that
dimension will bypass the hardware composer HAL and be handled only
by SurfaceFlinger.

Bug: 17701816
Change-Id: Ia6ca44dfd6a7a9bc0f054493d3f13006bc32fa14