Fix QtNative.checkSelfPermission()

Only return activity object when available and *not* when unavailable.

Task-number: QTBUG-55035
Task-number: QTBUG-50759
Change-Id: I60b1be528b2e4a3d630f4085b7642d74e18482bd
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>
Reviewed-by: BogDan Vatra <bogdan@kdab.com>

src/android/jar/src/org/qtproject/qt5/android/QtNative.java

@@ -456,7 +456,7 @@ public class QtNative
     }
 
     public static Context getContext() {
-        if (m_activity == null)
+        if (m_activity != null)
             return m_activity;
         return m_service;
     }