Unhide ImageDecoder

ImageDecoder is a new way to decode Bitmaps (or Drawables) that
streamlines common use cases of BitmapFactory.

Bug: 63909536
Bug: 63908092
Test: I0f36ce34c968fd7fae4d8edebabea3a421859615
      Ib2877276da8464b5f3eef0bbb848de202c90e97e
      I8d1672180d8325ae1caf44f0bbf41036b94e6253
      I405ebc86f7b6b019e0f040f1d4afe2e9d4354e5d
      Iaeb7c27bafb351932f0fabe59461ef50b1e2424a

Change-Id: Iee236ac73e0bc37ef6903a8150c0d2c84e5cf906