exynos4: hal: include: Exif.h: always keep two decimal places in macros

The APEX macros always rounded to int, losing too much precision. Ensure
that we always keep at least two decimal digits.

The more precise F-number comes from the value recorded by the camera
closed source driver.

Added a new macro APEX_SHUTTER_TO_EXPOSURE(x) (credit to tryptophane).

Change-Id: Ief43c15c6517d8dc48d4d4bc9e93df9eb01e6b91