Restore ObjectLevelLockingPolicy on ShaderManager
We are hitting a race condition otherwise. This is likely due to the fact that removing the shaders is done only once we have unloaded them from the graphics API which technically could happen in a separate thread than the scenegraph thread when running with Scene3D.
Change-Id: Ic914f3b305bf807081294a90250413a5d263f124
Fixes #239