Merge changes I888a268b,Ia5a3c0bb,I189841e4,I5ab8b0b0

* changes:
  Replace StringPiece with std::string_view in RegType.
  Replace StringPiece with std::string_view in OatFile.
  ART: Change ArtMethod::NumArgRegisters() signature.
  Remove dead String::Equals() overloads.