[104] Backdrop code review feedback (#29)

* [104] PR feedback.

* [104] PR feedback.

* [104] Some whitespace.

Author: willlarche

mdc_100_series/lib/backdrop.dart

@@ -146,16 +146,11 @@ class _BackdropState extends State<Backdrop>
   @override
   void didUpdateWidget(Backdrop old) {
     super.didUpdateWidget(old);
+    
     if (widget.currentCategory != old.currentCategory) {
-      setState(() {
-        _controller.fling(
-            velocity:
-              _frontLayerVisible ? -_kFlingVelocity : _kFlingVelocity);
-      });
+      _toggleBackdropLayerVisibility();
     } else if (!_frontLayerVisible) {
-      setState(() {
-        _controller.fling(velocity: _kFlingVelocity);
-      });
+      _controller.fling(velocity: _kFlingVelocity);
     }
   }