Reduce Libvpx output buffers to 8, to reduce the chances of out of memory errors.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=184317120

extensions/vp9/src/main/java/com/google/android/exoplayer2/ext/vp9/LibvpxVideoRenderer.java

@@ -98,7 +98,7 @@ public class LibvpxVideoRenderer extends BaseRenderer {
    * The number of output buffers. The renderer may limit the minimum possible value due to
    * requiring multiple output buffers to be dequeued at a time for it to make progress.
    */
-  private static final int NUM_OUTPUT_BUFFERS = 16;
+  private static final int NUM_OUTPUT_BUFFERS = 8;
   /**
    * The initial input buffer size. Input buffers are reallocated dynamically if this value is
    * insufficient.