build and use ffmpeg extension

pointing at my own repo for RNTP to enable extensions in exoplayer
new prepare script needs to be run to build ffmpeg

scripts/prepare-exoplayer-ffmpeg.sh

@@ -0,0 +1,27 @@
+#!/bin/bash
+set -e
+
+dir=$(dirname "$(readlink -f "$0")")
+root=$(realpath "$dir/..")
+
+FFMPEG_EXT_PATH="$root/submodules/ExoPlayer/extensions/ffmpeg/src/main/jni"
+HOST_PLATFORM="linux-x86_64"
+ENABLED_DECODERS=(alac)
+
+ndkLocalPath="$root/ndk/android-ndk-r21e"
+ndkDownloadZip="$root/ndk/ndk.zip"
+
+if [ -z "$NDK_PATH" ] && [ ! -e "$ndkLocalPath" ]; then
+    mkdir -p "$root/ndk"
+    wget https://dl.google.com/android/repository/android-ndk-r21e-linux-x86_64.zip -O "$ndkDownloadZip"
+    unzip -d "$root/ndk" "$ndkDownloadZip"
+    rm "$ndkDownloadZip"
+fi
+
+if [ -z "$NDK_PATH" ] && [ -e "$ndkLocalPath" ]; then
+    NDK_PATH="$root/ndk/android-ndk-r21e"
+fi
+
+cd "${FFMPEG_EXT_PATH}"
+./build_ffmpeg.sh "${FFMPEG_EXT_PATH}" "${NDK_PATH}" "${HOST_PLATFORM}" "${ENABLED_DECODERS[@]}"
+"${NDK_PATH}/ndk-build" APP_ABI="armeabi-v7a arm64-v8a x86 x86_64" -j4