Fixes tests for Sprites and removes the 0.5f approximation
  - Adds a guard clause in the Sprite constructor for a null so that we can mock
    the value of density=1 when the test is not run in an Anroid environment.
  - Removes the 0.5f approximation in all density formulas (pixel * density +
    0.5f), so we can keep most widths and heights as ints instead of floats.