This CL contains the following changes:
- Add a variation selector argument into getFamilyForChar to be able to
  select fonts which support variation selector.
- In case no fonts support the codepoint and variation selector pair,
  add a fallback rule which selects font family with ignoring variation
  selector.
- Change FontCollection::itemize to not change the font family
  immediately preceding a variation selector.
- Introduce unit tests for variation selectors.

With this CL, TextView can render the variation selectors correctly.

Bug: 11256006
Change-Id: I22ce0e9eadc941f84e3a9b23462f194e51dd7180