[DocsUI, Search]: Fix testSearchV2LastModifiedDropdownVisibility

check(doesNotExist()) and check(matches(withEffectiveVisibility(
ViewMatchers.Visibility.GONE))) and not the same. The first one checks
that the view is null, the other that the view is not null, but with
visiblility set to GONE.

Bug: 425591387
Test: m DocumentsUIGoogleTests:com.android.documentsui.SearchViewUiTest
Flag: com.android.documentsui.flags.use_search_v2_read_only
Change-Id: I308cd5850f1534e5be2861214b72e72202f4bef1