Don't do layout in hasGlyph for codepoint + vs

In the case where the input is a base codepoint + a variation selector,
the code currently checks hasVariationSelector and then does a layout,
checking for a single glyph. However, HarfBuzz in some cases will change
the VS into a space glyph, so hasGlyph will return false.

This patch makes hasGlyph rely entirely on the hasVariationSelector
method in the case of a base codepoint + a VS

Bug: 27531970
Bug: 28182689
Change-Id: Id190c427149213509f2d779ec1aa330feb4b62d8