Replace StringPiece with std::string_view in Signature.

And also in Signature-related code. Remove the function
DexFile::CreateSignature() which was used only in a test
as the test can use method searching functions that take
std::string_view instead.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 123750182
Change-Id: I3f24c8f4f677e2e40503dbab347df1eb031b4132