[Toolkit.Samples] Moved KeyboardManager creation from LoadContent to game constructor

Having KeyboardManager created in LoadContent may create two instances (if content will be recreated - on device reset, for example - I am not sure if this is applicable to Toolkit.Game).

The input managers were designed to be instantiated only once - two instances may not work correctly and will slow down the app.

I'm thinking if there can be created a mechanism of component instantiation, that will make sure it is put in the right place - but it may be over-engineering at this time. What do you think?