If U+FE0E is appended to the emoji code point, the glyph should have a
text presentation.
On the other hand, if U+FE0F is appended to the emoji code point, the
glyph should have an emoji presentation.

Bug: 11256006
Change-Id: I5187d44500b13a138e7ffbcf2c72e2da06374c8c

The behavior hasn't changed much yet: all languages are ignored for
rendering text, except the very first supported language.

Change-Id: I1695fb985927ae5e28e4f59c1b531e4993af8688

This CL introduces new method hasVariationSelector into FontFamily but it
is not used in production code. So no behavior changes are expected.

This CL contains the following changes:
- Introduce hasVariationSelector which returns true if the corresponding
  font has a glyph for a code point and variation selector pair.
- Introduce purgeHbFontCache since hb_face_t won't be released by
  keeping hb_font_t.
- Introduce unit tests with self-built font.

Change-Id: I659a6d03d9ec446b409e1fba2758452abb9f44fa