[DocsUI] Clear selection properly in tests

Follow up to ag/33285246 which finds the cancel/clear selection
button within the now selection_bar rather than the toolbar.

Bug: 412895530
Test: atest FileCopyUiTest
Test: atest FilesActivityUiTest
Test: atest InternalStorageUiTest
Test: atest PeekUiTest
Test: atest SearchViewUiTest
Flag: com.android.documentsui.flags.use_material3
Change-Id: I557463a4b45f26905b720d53875132c343205feb