Move the download of ndk to build_android.sh script file.

.travis.yml

@@ -4,7 +4,7 @@ cache:
     - $HOME/.ccache
     - $HOME/.cache/pip
     - $TRAVIS_BUILD_DIR/build/third_party
-    - $HOME/android-toolchain-gcc
+    - $TRAVIS_BUILD_DIR/build_android/third_party
 sudo: required
 dist: trusty
 os:
@@ -35,12 +35,6 @@ addons:
       - ccache
 before_install:
   - if [[ "$JOB" == "check_style" ]]; then sudo ln -s /usr/bin/clang-format-3.8 /usr/bin/clang-format; fi
-  - if [[ "JOB" == "build_android" ]]; then
-      mkdir -p $HOME/tmp/$JOB;
-      cd $HOME/tmp/$JOB; wget -q https://dl.google.com/android/repository/android-ndk-r14b-linux-x86_64.zip; unzip -q android-ndk-r14b-linux-x86_64.zip;
-      sh $HOME/tmp/$JOB/android-ndk-r14b/build/tools/make-standalone-toolchain.sh --arch=arm --platform=android-21 --install-dir=$HOME/android-toolchain-gcc;
-      cd $HOME; rm -rf $HOME/tmp/$JOB;
-    fi
   # Paddle is using protobuf 3.1 currently. Protobuf 3.2 breaks the compatibility. So we specify the python
   # protobuf version.
   - pip install numpy wheel 'protobuf==3.1' sphinx==1.5.6 recommonmark sphinx-rtd-theme==0.1.9 virtualenv pre-commit requests==2.9.2 LinkChecker

paddle/scripts/travis/build_android.sh

 #!/bin/bash
 set -e
 
+ANDROID_STANDALONE_TOOLCHAIN=$HOME/android-toolchain-gcc
+TMP_DIR=$HOME/$JOB/tmp
+mkdir -p $TMP_DIR
+cd $TMP_DIR
+wget -q https://dl.google.com/android/repository/android-ndk-r14b-linux-x86_64.zip
+unzip -q android-ndk-r14b-linux-x86_64.zip
+chmod +x $TMP_DIR/android-ndk-r14b/build/tools/make-standalone-toolchain.sh
+$TMP_DIR/android-ndk-r14b/build/tools/make-standalone-toolchain.sh --force --arch=arm --platform=android-21 --install-dir=$ANDROID_STANDALONE_TOOLCHAIN
+cd $HOME
+rm -rf $TMP_DIR
+
 # Create the build directory for CMake.
 mkdir -p $TRAVIS_BUILD_DIR/build_android
 cd $TRAVIS_BUILD_DIR/build_android
 
 # Compile paddle binaries
 cmake -DCMAKE_SYSTEM_NAME=Android \
-      -DANDROID_STANDALONE_TOOLCHAIN=$HOME/android-toolchain-gcc \
+      -DANDROID_STANDALONE_TOOLCHAIN=$ANDROID_STANDALONE_TOOLCHAIN \
       -DANDROID_ABI=armeabi-v7a \
       -DANDROID_ARM_NEON=ON \
       -DANDROID_ARM_MODE=ON \