Fix string getter implementation

Fix string getter implementation

In the previous implementation, the file does not have the null-terminated string
if the data is string type. After refactored the implementation, the file has
the null-terminated string. The preference API checks whether the last character
is the null-terminated string or not. And, if it's not, the null-terminated string
will be added. This patch is for backward compatibility.

Change-Id: Ia0a50d008be7f66616410c80d98b963dc36e68ad
Signed-off-by: Hwankyu Jhun <h.jhun@samsung.com>