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
diff --git a/media/libstagefright/timedtext/TimedTextSRTSource.cpp b/media/libstagefright/timedtext/TimedTextSRTSource.cpp
index eac23ba..2ac1e72 100644
--- a/media/libstagefright/timedtext/TimedTextSRTSource.cpp
+++ b/media/libstagefright/timedtext/TimedTextSRTSource.cpp
@@ -36,6 +36,9 @@
         : mSource(dataSource),
           mMetaData(new MetaData),
           mIndex(0) {
+    // TODO: Need to detect the language, because SRT doesn't give language
+    // information explicitly.
+    mMetaData->setCString(kKeyMediaLanguage, "und");
 }
 
 TimedTextSRTSource::~TimedTextSRTSource() {
@@ -46,14 +49,10 @@
     if (err != OK) {
         reset();
     }
-    // TODO: Need to detect the language, because SRT doesn't give language
-    // information explicitly.
-    mMetaData->setCString(kKeyMediaLanguage, "");
     return err;
 }
 
 void TimedTextSRTSource::reset() {
-    mMetaData->clear();
     mTextVector.clear();
     mIndex = 0;
 }