[Android] Add ReactiveActivity ctors for JNI ownership transfer

[jonfuller]

In some circumstances, the Xamarin Android framework already has a reference to an object, but needs to create it's peer MCW. In these instances, the constructor that takes (IntPtr, JniHandleOwnership) is called.

This is detailed here: Java Activation

Also, an SO answer from @jonpryor discussing this a bit: http://stackoverflow.com/a/10603714/335675

This change adds the aforementioned constructors to both the ReactiveActivity and ReactiveActivity<TViewModel> classes.

[anaisbetts]

Into this, do we need this for Fragments too?

[jonpryor]

Into this, do we need this for Fragments too?

Probably. Fragments can be created by Android via Reflection unless you set Fragment.RetainInstance to true. If Android is creating your Fragment instance first, you need the Fragment(IntPtr, JniHandleOwnership) constructor.

Also, be very careful when using generic fragments. Creating instances of generic managed types from Java is not supported.

[jonfuller]

Good idea.

I can go ahead and add those constructors for the fragment classes as well, if that helps things out.

Just let me know.

Thanks!

[anaisbetts]

@jonfuller Please do!

[jonfuller]

Fragment constructors added! Thanks for the input/feedback @paulcbetts and @jonpryor .

[anaisbetts]

Thanks @jonfuller!