[DocsUI, Search]: Fix result duplicates.

When searching everywhere, we were including external storage provider
and media and downloads proviers. Those last two (or more like 4)
replicate results that can be located via external storage provider.
This is now removed, for search everywhere. Various tests are fixed and
expanded.

Bug: 416107009
Test: DocumentsUIUnitTests:com.android.documentsui.base.SearchLoaderTest
Test: DocumentsUIUnitTests:com.android.documentsui.base.SearchViewManagerTest
Flag: com.android.documentsui.flags.use_search_v2_read_only

Change-Id: Id55fc56c318ddbfa06fe953953fbd2f46b092449