Fix NinePatchDrawable handling of density changes

Scaling is now manually applied when the canvas density is not set.

Removes set/getNinePatch accessors that were added in N, since these
are not used and setting the nine patch directly will break padding.
Also removes local NinePatch, which was not necessary.

Bug: 27038114
Change-Id: Ie8b2e485b54d1ed1521081b329a09915fd0a95c1