Support InMemoryDexClassLoader in ClassLoaderContext

Add new class loader tag IMC to class loader context spec which
represents InMemoryDexClassLoader. A special case is required to not
attempt to open its dex files as the dex location does not correspond
to a real file path. This is achieved by setting load-attempted variable
to 'true' when encountering IMC whilst parsing a spec. Context with IMC
can still have opened dex files if it was created from an existing class
loader.

Bug: 72131483
Test: m test-art-host-gtest-class_loader_context_text
Change-Id: Ic64065819018a1e56dee0f65405d26beb8fd7bbd