Favor delegation over composition

* Renaming FlowOwnerView to ScreenConductor. It's not a specific view anymore (frame layout or whatever) and could be reused for any kind of container. This enables easily using several flow owners.
* Introducing CanShowScreen which has to be implemented by the flow view. Most likely you'll just want to delegate to a ScreenConductor, but you could also do your own thing.
* DemoActivity directly accesses the main flow instead of the boilerplate calls to up / back (with different names each time)
* Got rid of the weird retrieval code for MainView. Don't do this.
Merge pull request #40 from square/py/compose_flow_owner_view

Favor delegation over composition