Add DEPTH image formats, support in ImageReader

- Add an explicit mapping between public ImageFormat/
  PixelFormat enums and internal HAL format/dataspace.
- Add DEPTH16 and DEPTH_POINT_CLOUD formats
- Wire up mapping layer to ImageReader to support depth
  formats

Change-Id: I8197eccef900cc91baddcfcb934ccd4d8c972eff