GVRVideoSceneObject instance is not fully ready for use immediately after the ctor returns

[liaxim]

A GVRVideoSceneObject can be created on any thread but its ctor creates an object on the GL thread. Between completion of the ctor call and this helper object being created, certain APIs would return null or have no effect. There is no way to know when the instance is fully ready and in any case it should be all transparent to end users.

[deepak-rawat]

@liaxim How about adding a callback function like setOnPreparedListener(OnPreparedListener) Just like android media player does