ExifInterface: Prevent DATETIME value from being set unproperly

Although the DATETIME and DATETIME_ORIGINAL tags are for different
purposes, the DATETIME value is set to the same value as
DATETIME_ORIGINAL tag's when loading the attributes of an image.
This CL prevents the problem by setting the value of DATETIME tag only
when it does not exist.

Bug: 62137632
Test: Passed ExifInterface CTS
Change-Id: Ic5c43c53cb363a782bcac20915d74ea534ec70e4