Change well known `ClassLoader` fields to `ArtField*`.

Note that the new field caching does not resolve the field's
type and does not initialize the class holding the field.
There is no reason why the `WellKnownClasses` should be
responsible for this type resolution and initialization.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Bug: 253570082
Change-Id: I692afdc891d161c813864543e47a2c02f93e93e8