[DocsUI,Search]: Improving recent view's recall.

Matching version v1 of search, we include Downloads in recent search.
This CL also adds a test to show that zip files can be located among
recent files.

Bug: 404651371
Test: atest DocumentsUIGoogleTests
Flag: com.android.documentsui.flags.use_search_v2_read_only
Change-Id: Ibd0b4274697d5edc916812400269669e351beb5b