Change Landmark's landmark_type, landmark_coordinate, and decoding_type to string

Change Landmark's landmark_type, landmark_coordinate, and decoding_type to string

A user can use the string while understanding purpose of
landmark_type, landmark_coordinate, and decoding_type metadata.
The string values are parsed and converted to enumeration type
inference_landmark_type_e, inference_landmark_coordinate_type_e,
and inference_landmark_decoding_type_type_e, respectively.

Change-Id: Ia3a4098213a712ffd1927838d7b9931d1edfcbd0
Signed-off-by: Tae-Young Chung <ty83.chung@samsung.com>