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