Better merge fix for empty state signal drawable

The cutout for drawing STATE_EMPTY picked up a fudge factor in the merge
due to the rounded edges changing the size of the triangle. This change
parameterizes the drawing of the cutout based on the placement of the
rounded edges.

Test: visual
Bug: 62905681
Change-Id: Iba2bc0aa240f6bdfb285c2c6535b656bcacbd579