Revert^2: 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.

Previously this CL would fail on target because IMC classpath was not
being replaced with "<unknown>" and context matching failed. This was
only a problem on target because
OatFile::ResolveRelativeEncodedDexLocation ignores non-absolute
locations on host.

This reverts commit 93d99f36.

Test: m test-art-gtest-class_loader_context_test
Bug: 72131483
Change-Id: I27cbfa69c24d412cc1b6bcce88218cc95e324ef5