Update exoplayerv2 vp9 extension.

Reset RenderedFirstFrame when setting OuputBufferRenderer.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=139100783

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

@@ -543,6 +543,7 @@ public final class LibvpxVideoRenderer extends BaseRenderer {
     if (this.outputBufferRenderer == outputBufferRenderer) {
       return;
     }
+    renderedFirstFrame = false;
     this.outputBufferRenderer = outputBufferRenderer;
     surface = null;
     outputMode = (outputBufferRenderer != null) ? VpxDecoder.OUTPUT_MODE_YUV