Add explicit GC.Collect in Sample

There's a large image loaded as the backdrop.  Let's help the GC a bit by collecting after we've done so.  This should help Bugzilla #40036.

design/samples/Cheesesquare/CheeseDetailActivity.cs

@@ -50,6 +50,8 @@ namespace Cheesesquare
 
             var r = Cheeses.GetRandomCheeseBackground ();
             imageView.SetImageResource (r);
+
+            GC.Collect ();
         }
             
         public override bool OnCreateOptionsMenu(IMenu menu)