WelcomeActivity: do nothing on back press

Devices with a hardware back button, such as atv remotes, can press back
on the welcome screen and trigger a transition to a waiting screen then
back to welcome. This should not be allowed.

Change-Id: I2273d0d74fb1ef018074d4facc8e3793e7470f6f

src/org/lineageos/setupwizard/WelcomeActivity.java

@@ -60,6 +60,10 @@ public class WelcomeActivity extends SubBaseActivity {
         }
     }
 
+    @Override
+    public void onBackPressed() {
+    }
+
     @Override
     protected int getLayoutResId() {
         return R.layout.welcome_activity;