ui: Handle back pressed

Adapt to new behaviour of onBackPressed in app-compat 24.

Change-Id: Icc8c9801d217146e25c2e2050e45081cfa06e51e
Tuleap: #1087

ring-android/app/src/main/java/cx/ring/client/HomeActivity.java

@@ -423,7 +423,7 @@ public class HomeActivity extends AppCompatActivity implements LocalService.Call
             return;
         }
 
-        super.onBackPressed();
+        finish();
     }
 
     private void popCustomBackStack() {