[DocsUI] Add clear_selection to proguard

Follow up to ag/33406606 which uses clear_selection in tests.

Bug: 408898634
Fix: 415967495
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: I6802d575d76e0832a34eeb9fceb3057713132030