media: use size_t for integer iterator to Vector::size()

Change-Id: I0a744dc7815a86a993df9b0623440be620ec8903
diff --git a/services/audioflinger/Tracks.cpp b/services/audioflinger/Tracks.cpp
index fccc7b8..b59333f 100644
--- a/services/audioflinger/Tracks.cpp
+++ b/services/audioflinger/Tracks.cpp
@@ -895,7 +895,7 @@
 
 void AudioFlinger::PlaybackThread::Track::triggerEvents(AudioSystem::sync_event_t type)
 {
-    for (int i = 0; i < (int)mSyncEvents.size(); i++) {
+    for (size_t i = 0; i < mSyncEvents.size(); i++) {
         if (mSyncEvents[i]->type() == type) {
             mSyncEvents[i]->trigger();
             mSyncEvents.removeAt(i);