LiveSession: Fixed a bug that set invalid metadata key for subtitles.

An error occurs in nuplayer due to invalid key value setting in livesession.
The "track-index" key has been modified in aosp p-os.

Test: play hls streaming with webvtt subtitles.

Signed-off-by: beattyblue.kim <beattyblue.kim@lge.com>

Change-Id: I494889becc6497cd3a747bd82d57fcc755a3da89