camera: add exposure-time value in EXIF

Bug: 7234966

for tuning sensor brightness, sensor vendor want to extract ev value
from picture via exit header. now mako can support ev value.

Change-Id: Ie27586be4f23b9001da767dcb09cc6d0f4f6a5b2
Signed-off-by: Iliyan Malchev <malchev@google.com>
2 files changed