[DocsUI, Search]: Use class name in logging.

To make more obvious which version of the loader is calling based
method, we change BaseFileLoader to this::class.simpleName.

Bug: 381326491
Test: m DocumentsUIGoogle
Flag: com.android.documentsui.flags.use_search_v2_read_only
Change-Id: Ib8cb36bfe095a6407b649d743a0a5588561cb788