[DocsUI, Search]: Increase test coverage.

Adds a test for checking that hidden files are shown as per query option
field.

Bug: 427073832
Test: atest DocumentsUIUnitTests
Flag: com.android.documentsui.flags.use_search_v2_read_only
Change-Id: I3f75b84af61eca926ba23d453bc68e3c284c54d9