Minor refactoring of `ClassLinker::LinkMethods()`.

Rename `LinkInterfaceMethodsHelper` to `LinkMethodsHelper`
and move `LinkVirtualMethods()` and `LinkInterfaceMethods()`
as well as the default translations map to the helper class.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Change-Id: Id66d1ed70addc8aa1bf7bb47b89e85710de20fb5