This change makes us reach to fragment manager only when screen container is attached to window (and therefore we are sure that the current activity is react one). The problem reported in #33 was due to the fact we'd do that when container is instantiated which does not necessarily mean that react activity is in foreground. We didn't need to actually access fragment manager while not in foreground so this change removes fragmentManager as a member and we get it directly from context when needed.
Supposedly fixes#33
This change adds an ability for screen container on Android to apply the correct mechanism for transparent layer blending. This is specifically important as screens are usually a complex views that may displays many layers and while transitioning often opacity is used to animate these. When we detect screen transitioning we (a) turn on offscreen alpha compositing (which makes the opacity being applied for the whole screen layer at once instead of making all the children semi-transparent) and also (b) turn on hardware layer that makes offscreen compositing render to GPU (which is both faster and consumes only GPU memory).
In addition to that change we also need to disable wrapping Screen's children with View, as in such a case opacity is applied on the underlying View instead of a Screen. That workaround has been added because of a bug in Animated library and fixed in RN 0.57+
