[AssetAtlas] Avoid packing 1-pixel width or height asset into AssetAtlas

Symptom: For preloaded asset which has 1-pixel width (or 1-pixel height),
         sampling the atlas texture will get a color mixed with the surrounding
         pixels' color.
         If we have padding in the atlas texture , sampling will mix the
         padding's black color and result in an obvious defect.

Solugion: We avoid packing 1-pixel width (or 1-pixel height) asset into
          AssetAtlas.

Change-Id: I765673dfa13cf3f3709196cf15f35d3493d1edd8