Bug fix: set 'und' as default metadata language info for srt.

o Previously, it leaves language code empty and it is inconsistent with
  other code which gives 'und'. As a result, selected SRT track returned
  empty language info. With this fix, it returns 'und'.

Bug: 7227230

TESTED=runtest -d cts-media -c android.media.cts.MediaPlayerTest -m
testGetTrackInfo

Change-Id: I225848f029637dd782c376e0d770dddd5c849550