Don't attempt to reuse ImagePresenters in top level interface.

Change-Id: Ic8c82958c4b0e28c6d1df3d950901be034cbc7e3
Fix NPEs with optional loaders in BaseBitmapLoadTask.

Change-Id: I4cfec042db8bb30318f61c396853d75d06a7d2f7
Add convenience methods for retrieving common model loaders.

Change-Id: Ia83a51f9a8e32363e27af35493c947f1757cd8a6