Yield the omx threads so that file writer threads have a chance to retrieve the output buffers once they become ready

bug - 3252228

Change-Id: Iab7cf1c9a87cce7b814c63b42b21fcd6a7314b36
diff --git a/media/libstagefright/OMXCodec.cpp b/media/libstagefright/OMXCodec.cpp
index b954be7..43e4e97 100644
--- a/media/libstagefright/OMXCodec.cpp
+++ b/media/libstagefright/OMXCodec.cpp
@@ -2029,6 +2029,9 @@
 
                 mFilledBuffers.push_back(i);
                 mBufferFilled.signal();
+                if (mIsEncoder) {
+                    sched_yield();
+                }
             }
 
             break;