[DocsUI] Update clearSelection() method to use "clear selection" text

Follow up to ag/33384387 which changed the content description text from
"Cancel" to "Clear selection". The corresponding test method
clearSelection() needs to be updated.

Bug: 408898634
Test: atest FileCopyUiTest
Test: atest FilesActivityUiTest
Test: atest InternalStorageUiTest
Test: atest PeekUiTest
Test: atest SearchViewUiTest
Test: atest PickActivityTest
Flag: com.android.documentsui.flags.use_material3
Change-Id: I06c6dde825c8c42e9f3c466bf1d895f6de7f7a78