ART: Fix Unstarted Runtime Class.forName

We really only support the boot classloader, and only in the null
denotation. Otherwise the class-linker would want to install a
class table into the fake BootClassLoader.

Add tests.

Bug: 34956610
Test: m test-art-host-gtest-unstarted_runtime_test
Change-Id: I47e284fbd17eb8d33665d2788afcbcc3d09d3d2e