This CL does following things:
- Extract hb_face_t object cache mechanism from Layout.cpp to be able to
use it from other cpp file, especially from FontFamily.cpp.
To address Bug 11256006 and Bug 17759267, need to touch
hb_face_t from FontFamily.
- Make hb_face_t cache mechanism thread-safe.
- Add unit tests for HbFaceCache test cases.
Bug: 11256006
Bug: 17759267
Change-Id: Ic183634ef34326793bd9a32167236611d0af34d6
