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
7 files changed