Fix NuPlayer assertion on failure to create AudioTrack

Under heavy media load or monkey/stress testing, more than
32 AudioTracks may be created or memory resources may be scarce.
Remove the assertion on failure to create AudioTrack
and signal MEDIA_ERROR.

Bug: 17319843
Change-Id: I5d4e200b5f50d800046851a33e035cdc6ff10075
diff --git a/media/libmediaplayerservice/nuplayer/NuPlayerDecoderBase.h b/media/libmediaplayerservice/nuplayer/NuPlayerDecoderBase.h
index 5feb6a1..6732ff4 100644
--- a/media/libmediaplayerservice/nuplayer/NuPlayerDecoderBase.h
+++ b/media/libmediaplayerservice/nuplayer/NuPlayerDecoderBase.h
@@ -29,7 +29,7 @@
 struct MediaBuffer;
 
 struct NuPlayer::DecoderBase : public AHandler {
-    DecoderBase();
+    DecoderBase(const sp<AMessage> &notify);
 
     void configure(const sp<AMessage> &format);
     void init();
@@ -71,6 +71,10 @@
     void onRequestInputBuffers();
     void scheduleRequestBuffers();
     virtual void doRequestBuffers() = 0;
+    virtual void handleError(int32_t err);
+
+    sp<AMessage> mNotify;
+    int32_t mBufferGeneration;
 
 private:
     enum {