finish typings for iron-overlay-behavior managers

[aomarks]

• IronScrollManager is missing currentLockingElement, because it is a getter, and the Analyzer namespace scanner does not handle getters. We can either fix https://github.com/Polymer/polymer-analyzer/issues/866, or use the addReferences setting in gen-tsd.json to augment the namespace with a manually written version. Augmenting will also require fixing https://github.com/Polymer/gen-typescript-declarations/issues/86, however, because otherwise the manually written file will get deleted every time the typings are re-generated.

• IronOverlayManager is a singleton instance of a class. The class itself is @private. Also the class is created by directly writing to .prototype, which Analyzer doesn't really understand (at least not without a lot of annotations). For this, we probably just want to manually write all of the typings and add them with addReferences.