Use reference type to reduce copy operation

Use reference type to reduce copy operation

- Issue:
When the code iterates the list, some code repeats the unnecessary copy
operation.

- Solution:
Normally, reference type is used when the list is iterated. However,
some code did not use reference type, so those code would repeat
unnecessary copy operation. So, this patch fixes those types to
reference. Through this patch, unnecessary copy operation will be
removed.

Change-Id: Ib64f990c0f33ba5eea2977a80db4765f539c5b04
Signed-off-by: Suyeon Hwang <stom.hwang@samsung.com>