Thread-safety issue with GVRSceneObject.removeChildObject

[liaxim]

Erik has observed this (bug 20321):

Fixes a race condition between removing a scene object on a non-GL thread and rendering that object which can result in the object being rendered with only its local transform. Visually, this appears as the object in the wrong place, with the wrong orientation, for a single frame.

The issue is probably more general.