Image viewer.

- Added an Activity for viewing individual images clicked from a page.
  Eventually the activity may be adapted to accommodate a scrollable
  gallery of images.
- The activity has a Toolbar with a gradient background, as well as a
  blank container for description and license information, and a stub menu
  item for displaying additional (exif) information.
- Created task for retrieving metadata for an individual File page.

Change-Id: I739d68860e957b84a7a927e089db10b3695d9598