frameworks/base: surfaceflinger: Enable dither if needed

SF is checking if dithering is required but not enabling it.
Enable for layer drawing and  also for screenshot for 16bpp
framebuffer only.

CRs-fixed: 333721
(cherry picked from commit 85c02ed97b4a1d0cc343a452959570b379928d59)

Change-Id: I599fe5b9683db208c8b32a1d3d395d5203f78fd3