vp9_extension: Update libyuv build configuration

Disable libjpeg dependency when building libyuv.

Partially fixes Github Issue #2661

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=154179126

extensions/vp9/README.md

@@ -49,7 +49,7 @@ git clone https://chromium.googlesource.com/libyuv/libyuv libyuv
 cd "${VP9_EXT_PATH}/jni/libvpx" && \
 git checkout tags/v1.6.1 -b v1.6.1 && \
 cd "${VP9_EXT_PATH}/jni/libyuv" && \
-git checkout e2611a73
+git checkout 996a2bbd
 ```
 
 * Run a script that generates necessary configuration files for libvpx:

extensions/vp9/src/main/jni/Android.mk

@@ -21,6 +21,7 @@ LIBYUV_ROOT := $(WORKING_DIR)/libyuv
 
 # build libyuv_static.a
 LOCAL_PATH := $(WORKING_DIR)
+LIBYUV_DISABLE_JPEG := "yes"
 include $(LIBYUV_ROOT)/Android.mk
 
 # build libvpx.so