Do not turn on dithering by default

Bug: 195281495
Test: Ief7025c21f365ae90fa63120f1cc82e0695901af

This is a partial revert of 47c51fe8/
I5d5d03e430af380e23016c6deba5eca46067a22b. The intent was to bring
today's rendering implementation in line with the old HWUI
implementation, which always dithered gradients by default. However,
when dithering is enabled, the Skia implementation dithers more than
just gradients (e.g. Bitmaps). Combined with poor performance of the
dithering algorithm on certain GPUs, this led to a performance
regression on key apps.

Change-Id: Id54121091e2cc47131dc9b5ae67bd638fbc005fc