Summary: - Made `EnumMapping` support generic value type - `ImageView.ScaleType` is a Java enum; the scaleType api uses this instead of int types. - Introduce descriptor for `ImageView` and allow getting/setting scaleType. Reviewed By: mweststrate Differential Revision: D26092725 fbshipit-source-id: ea3a4ee036b2b808f14277387d360318b7d25824