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
diff --git a/media/libstagefright/httplive/LiveSession.cpp b/media/libstagefright/httplive/LiveSession.cpp
index 7eff8eb..86872c5 100644
--- a/media/libstagefright/httplive/LiveSession.cpp
+++ b/media/libstagefright/httplive/LiveSession.cpp
@@ -445,7 +445,7 @@
                return -EAGAIN;
             };
             (*accessUnit)->meta()->setInt32(
-                    "trackIndex", mPlaylist->getSelectedIndex());
+                    "track-index", mPlaylist->getSelectedIndex());
             (*accessUnit)->meta()->setInt64("baseUs", mRealTimeBaseUs);
         } else if (stream == STREAMTYPE_METADATA) {
             HLSTime mdTime((*accessUnit)->meta());