NuPlayerSource: clarify getFormat API and fix its usage.

Test: manually run mediaplayer
Bug: 33818804
Change-Id: I1a4e551493fa9521b3b1159182d446e8a9e7d523
diff --git a/media/libmediaplayerservice/nuplayer/NuPlayerSource.h b/media/libmediaplayerservice/nuplayer/NuPlayerSource.h
index 5197167..6006730 100644
--- a/media/libmediaplayerservice/nuplayer/NuPlayerSource.h
+++ b/media/libmediaplayerservice/nuplayer/NuPlayerSource.h
@@ -77,7 +77,11 @@
     // an error or ERROR_END_OF_STREAM if not.
     virtual status_t feedMoreTSData() = 0;
 
+    // Returns non-NULL format when the specified track exists.
+    // When the format has "err" set to -EWOULDBLOCK, source needs more time to get valid meta data.
+    // Returns NULL if the specified track doesn't exist or is invalid;
     virtual sp<AMessage> getFormat(bool audio);
+
     virtual sp<MetaData> getFormatMeta(bool /* audio */) { return NULL; }
     virtual sp<MetaData> getFileFormatMeta() const { return NULL; }