Replace StringPiece with std::string_view in class.h.

Translate the std::string_view to StringPiece where
needed for comparing with Signature until we change
the Signature::operator==() to take std::string_view.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 123750182
Change-Id: I9699f13882353d84dc69e6b8e30d12dac4bace24