Move ClassLoaderContexts() method to UsesLibraryDependency interface.

This method logically belongs with other UsesLibraryDependency methods,
as it is used in the same context when computing class loader context.
Previously it was part of Dependency interface, which doesn't cover SDK
libraries. We do not currently have the necessary information for
SdkLibraryImport (so ClassLoaderContexts() returns nil for it), but we
do have it for SdkLibrary (via Library).

Bug: 132357300
Test: m nothing
Change-Id: I56437f260efc6a9af3f45b334e84e915244ccce1