libstagefright : Fix for grey frame displayed on Seek in HTTP Streaming

-If seek is signaled while cache read is in progress, this frame should
 not be added to the cache.
-Currently this frame is written to the cache and subsequently gets sent
 to the decoder after flush.
-If this frame is not a sync frame, it results in grey frame /artifacts
 at the output.
-Added code to release this buffer and prevent it from being pushed to
 the cache

Change-Id: I1809bec6ba7d943f3c45729ae03812521cda4098
CRs-Fixed: 251274