a11y Windows: Correctly announce tristate checkboxes

Make sure partially-checked Qt Quick check boxes are correctly
announced, i.e. as partially checked instead of unchecked. For at least
Controls.CheckBox and Controls.CheckBoxDelegate, checkStateMixed does
not imply checked to be set.

Task-number: QTBUG-146686
Change-Id: I70b99885a3d66653a8eb1f58862dd00acdbd70ec
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
(cherry picked from commit 9f5a139)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

Author: frankosterfeld

src/plugins/platforms/windows/uiautomation/qwindowsuiatoggleprovider.cpp

@@ -57,10 +57,10 @@ HRESULT STDMETHODCALLTYPE QWindowsUiaToggleProvider::get_ToggleState(ToggleState
     if (!accessible)
         return UIA_E_ELEMENTNOTAVAILABLE;
 
-    if (accessible->state().checked)
-        *pRetVal = accessible->state().checkStateMixed ? ToggleState_Indeterminate : ToggleState_On;
-    else
-        *pRetVal = ToggleState_Off;
+    if (accessible->state().checkStateMixed)
+        *pRetVal = ToggleState_Indeterminate;
+    else if (accessible->state().checked)
+        *pRetVal = ToggleState_On;
     return S_OK;
 }