Add provider support for metadata extraction

In this CL, I'm adding ability to retrieve image metadata. I've added
the class MetadataRetriever that can currently retrieve Exif data from
an image. This class is used withing FileSystemProvider in order to
return a given image's exif data.

Test: Hidden API
Bug: 62621970
Change-Id: Ibb329492df4db1a65f65722c6a52582c43de8fe5