Fix ArchiveUiTest

DirectoryListBot.findPlaceholderMessageTextView() scrolls if necessary
to make the expected message visible. Simplified the tests that use this
method.

Bug: 423769615
Flag: EXEMPT fixing tests
Test: atest DocumentsUIGoogleTests:com.android.documentsui.ArchiveUiTest
Test: atest DocumentsUIGoogleTests:com.android.documentsui.SearchViewUiTest
Test: atest DocumentsUIGoogleTests:com.android.documentsui.FilesActivityDefaultsUiTest
Change-Id: I7703ecd9e608a75814353cb69b12af86b910ea1c