Change ArtField::ProxyFindSystemClass() to lookup the class.

As the function is called from ArtField::LookupType(),
we should avoid calls that appear to allow type resolution
rather than plain lookup. The lookup should always succeed.

Also rename ArtField::LookupType() to LookupResolvedType()
to align with naming used in ClassLinker and ArtMethod.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Change-Id: I0a87347b5341575e47e0fdba6d58ade2543387c8