Fixes rotation in sample app. Revealed necessary API changes.

1. Drops Mortar#createRootActivityScope. It's really not practical for the
root scope to be an instance of MortarActivityScope, there's no sane way for
them to survive the life cycle without a parent scope in the Application.

2. Views are now required to call Presenter#dropView at the appropriate time.

3. In the sample, ContainerView replaced by a more convincing FlowOwner
(abstract presenter) and FlowOwnerView (abstract view).