Prevent autofill for username editor.

app/src/main/res/layout/username_edit_fragment.xml

@@ -27,6 +27,7 @@
         android:layout_marginEnd="16dp"
         android:hint="@string/UsernameEditFragment_username"
         android:imeOptions="actionDone"
+        android:importantForAutofill="no"
         android:inputType="text"
         android:maxLines="1"
         android:minHeight="56dp"