Merge changes I14c7cddd,I4a39d0e8,I3f24c8f4,I9699f138

* changes:
  Replace StringPiece with std::string_view in art/runtime/.
  Replace StringPiece with std::string_view in HashSet.
  Replace StringPiece with std::string_view in Signature.
  Replace StringPiece with std::string_view in class.h.