Avoid storing reference to ApplicationContext

[cmelchior]

Right now we store a reference in the ApplicationContext which should be avoided due to InstantRun.

Instead we could:

• Just get the reference to context.getFilesDir() and save that
• get AssetManager and save that
• Register the network receiver on init() without saving the context

I don't think we use it for anything else, but this approach would also work for any future additions.

Perhaps we could wrap it in a AndroidContext<Wrapper/Facade/Mediator> class ?

[zaki50]

Register the network receiver on init() without saving the context was done in https://github.com/realm/realm-java/pull/3529