[DocsUI, Search]: Run tests only when search v2 flags are enabled.

Due to the way libraries are compiled, it is possible for
FolderLoaderTest and SearchLoaderTest to be run without QueryOption
class being compiled in. For now, we are limiting these tests to be run
only when the flag is enabled.

Test: m DocumentsUIGoogle
Bug: 397339113
Flag: com.android.documentsui.flags.use_search_v2_rw
Change-Id: I86820b0e3e3b9381ebc5d9fcfe0ce279b5738424