Clean up FFmpeg extension instructions.

Also add instructions for building arm64-v8a and x86.

Issue: #2561

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=152141270

extensions/ffmpeg/README.md

@@ -31,21 +31,12 @@ FFMPEG_EXT_PATH="${EXOPLAYER_ROOT}/extensions/ffmpeg/src/main"
 NDK_PATH="<path to Android NDK>"
 ```
 
-* Fetch and build FFmpeg. For example, to fetch and build for armv7a:
+* Fetch and build FFmpeg. For example, to fetch and build for armeabi-v7a,
+  arm64-v8a and x86 on Linux x86_64:
 
 ```
-cd "${FFMPEG_EXT_PATH}/jni" && \
-git clone git://source.ffmpeg.org/ffmpeg ffmpeg && cd ffmpeg && \
-./configure \
-    --libdir=android-libs/armeabi-v7a \
-    --arch=arm \
-    --cpu=armv7-a \
-    --cross-prefix="${NDK_PATH}/toolchains/arm-linux-androideabi-4.9/prebuilt/linux-x86_64/bin/arm-linux-androideabi-" \
+COMMON_OPTIONS="\
     --target-os=android \
-    --sysroot="${NDK_PATH}/platforms/android-9/arch-arm/" \
-    --extra-cflags="-march=armv7-a -mfloat-abi=softfp" \
-    --extra-ldflags="-Wl,--fix-cortex-a8" \
-    --extra-ldexeflags=-pie \
     --disable-static \
     --enable-shared \
     --disable-doc \
@@ -57,22 +48,56 @@ git clone git://source.ffmpeg.org/ffmpeg ffmpeg && cd ffmpeg && \
     --disable-postproc \
     --disable-avfilter \
     --disable-symver \
+    --disable-swresample \
     --enable-avresample \
     --enable-decoder=vorbis \
     --enable-decoder=opus \
     --enable-decoder=flac \
-    --enable-decoder=alac \
+    " && \
+cd "${FFMPEG_EXT_PATH}/jni" && \
+git clone git://source.ffmpeg.org/ffmpeg ffmpeg && cd ffmpeg && \
+./configure \
+    --libdir=android-libs/armeabi-v7a \
+    --arch=arm \
+    --cpu=armv7-a \
+    --cross-prefix="${NDK_PATH}/toolchains/arm-linux-androideabi-4.9/prebuilt/linux-x86_64/bin/arm-linux-androideabi-" \
+    --sysroot="${NDK_PATH}/platforms/android-9/arch-arm/" \
+    --extra-cflags="-march=armv7-a -mfloat-abi=softfp" \
+    --extra-ldflags="-Wl,--fix-cortex-a8" \
+    --extra-ldexeflags=-pie \
+    ${COMMON_OPTIONS} \
+    && \
+make -j4 && make install-libs && \
+make clean && ./configure \
+    --libdir=android-libs/arm64-v8a \
+    --arch=aarch64 \
+    --cpu=armv8-a \
+    --cross-prefix="${NDK_PATH}/toolchains/aarch64-linux-android-4.9/prebuilt/linux-x86_64/bin/aarch64-linux-android-" \
+    --sysroot="${NDK_PATH}/platforms/android-21/arch-arm64/" \
+    --extra-ldexeflags=-pie \
+    ${COMMON_OPTIONS} \
+    && \
+make -j4 && make install-libs && \
+make clean && ./configure \
+    --libdir=android-libs/x86 \
+    --arch=x86 \
+    --cpu=i686 \
+    --cross-prefix="${NDK_PATH}/toolchains/x86-4.9/prebuilt/linux-x86_64/bin/i686-linux-android-" \
+    --sysroot="${NDK_PATH}/platforms/android-9/arch-x86/" \
+    --extra-ldexeflags=-pie \
+    --disable-asm \
+    ${COMMON_OPTIONS} \
     && \
-make -j4 && \
-make install-libs
+make -j4 && make install-libs && \
+make clean
 ```
 
-* Build the JNI native libraries. Repeat this step for any other architectures
-  you need to support.
+* Build the JNI native libraries, setting `APP_ABI` to include the architectures
+  built in the previous step. For example:
 
 ```
 cd "${FFMPEG_EXT_PATH}"/jni && \
-${NDK_PATH}/ndk-build APP_ABI=armeabi-v7a -j4
+${NDK_PATH}/ndk-build APP_ABI="armeabi-v7a arm64-v8a x86" -j4
 ```
 
 * In your project, you can add a dependency on the extension by using a rule

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

@@ -32,14 +32,9 @@ LOCAL_SRC_FILES := ffmpeg/android-libs/$(TARGET_ARCH_ABI)/$(LOCAL_MODULE).so
 include $(PREBUILT_SHARED_LIBRARY)
 
 include $(CLEAR_VARS)
-LOCAL_MODULE    := libswresample
-LOCAL_SRC_FILES := ffmpeg/android-libs/$(TARGET_ARCH_ABI)/$(LOCAL_MODULE).so
-include $(PREBUILT_SHARED_LIBRARY)
-
-include $(CLEAR_VARS)
 LOCAL_MODULE := ffmpeg
 LOCAL_SRC_FILES := ffmpeg_jni.cc
 LOCAL_C_INCLUDES := ffmpeg
-LOCAL_SHARED_LIBRARIES := libavcodec libavresample libavutil libswresample
+LOCAL_SHARED_LIBRARIES := libavcodec libavresample libavutil
 LOCAL_LDLIBS := -Lffmpeg/android-libs/$(TARGET_ARCH_ABI) -llog
 include $(BUILD_SHARED_LIBRARY)