exit app on back pressed in tutorial (#3597)

* exit app on back pressed in tutorial

* exit app from ongoing tutorial

* exit app on back press in tutorial

Author: 318anushka

app/src/main/java/fr/free/nrw/commons/WelcomeActivity.java

@@ -86,7 +86,11 @@ public void onBackPressed() {
         if (pager.getCurrentItem() != 0) {
             pager.setCurrentItem(pager.getCurrentItem() - 1, true);
         } else {
-            finish();
+            if (defaultKvStore.getBoolean("firstrun", true)) {
+                finishAffinity();
+            } else {
+                super.onBackPressed();
+            }
         }
     }