Move descriptor hashing to `mirror::Class`. We may want to reuse the hashing for image classes which are not closely related with the `ClassTable` where the hashing was previously. Test: m test-art-host-gtest Test: testrunner.py --host --optimizing Bug: 181943478 Change-Id: Icf1b51085829509f58e7685dc1e9cf2b0583d107