Check stream type in AudioFlinger::createTrack

A bad parameter to AudioFlinger::createTrack could cause mediaserver to crash.

Other AudioFlinger stream type cleanup:
 - Simplify range check for audio_stream_type_t
 - Add comment about mStreamTypes array initialization.

Change-Id: Ia33aa1cce0fdd694b08d9288816ffc097a9543d0
diff --git a/services/audioflinger/AudioFlinger.h b/services/audioflinger/AudioFlinger.h
index cc7072c..9d1d862 100644
--- a/services/audioflinger/AudioFlinger.h
+++ b/services/audioflinger/AudioFlinger.h
@@ -799,7 +799,7 @@
         status_t    dumpTracks(int fd, const Vector<String16>& args);
 
         SortedVector< sp<Track> >       mTracks;
-        // mStreamTypes[] uses 1 additionnal stream type internally for the OutputTrack used by DuplicatingThread
+        // mStreamTypes[] uses 1 additional stream type internally for the OutputTrack used by DuplicatingThread
         stream_type_t                   mStreamTypes[AUDIO_STREAM_CNT + 1];
         AudioStreamOut*                 mOutput;
         float                           mMasterVolume;