InMemoryDexClassLoader in ClassLoaderContext follow-up

Address follow-up comments on Ic64065819018a1e56dee0f65405d26beb8fd7bbd.
In particular, the classpath elements of IMC are replaced with
"<unknown>" magic value to make it clear that the dex location is bogus,
and a clarifying comment is added.

Test: m test-art-gtest-class_loader_context_test
Bug: 72131483
Change-Id: I0225c288a07af589a3f3d85a69ef908eeab38cb2
2 files changed