[DocsUI] Close SysUI windows before DocumentsUI tests start running

Test devices can start in an unclean state where SysUI windows or
popups, like the wallpaper popup, end up obscuring UiAutomator's
view of DocumentsUI, causing tests to fail.

Add a helper before each test runs to close any open windows that
don't belong to DocumentsUI.

Flag: EXEMPT bugfix
Bug: 424324026
Test: atest DocumentsUIGoogleTests:com.android.documentsui.RenameDocumentUiTest
Change-Id: I505beff31713983a8fbab14b898e020fcce37771