update ndk url (#1562)

Dockerfile

@@ -28,11 +28,11 @@ RUN ln -s clang-format-5.0 /usr/bin/clang-format
 RUN pip install -i https://pypi.tuna.tsinghua.edu.cn/simple --upgrade pip
 RUN pip install -i https://pypi.tuna.tsinghua.edu.cn/simple wheel
 RUN pip install -i https://pypi.tuna.tsinghua.edu.cn/simple pre-commit
-RUN cd /tmp && curl -O http://mirrors.neusoft.edu.cn/android/repository/android-ndk-r17b-linux-x86_64.zip
+RUN cd /tmp && curl -O https://dl.google.com/android/repository/android-ndk-r17c-linux-x86_64.zip
 RUN curl -O https://mms-res.cdn.bcebos.com/cmake-3.10.3-Linux-x86_64.tar.gz && \
         tar xzf cmake-3.10.3-Linux-x86_64.tar.gz && \
         mv cmake-3.10.3-Linux-x86_64 /opt/cmake-3.10 && \
         mv /usr/bin/cmake /usr/bin/cmake.bak && ln -s /opt/cmake-3.10/bin/cmake /usr/bin/cmake && \
         mv /usr/bin/ccmake /usr/bin/ccmake.bak && ln -s /opt/cmake-3.10/bin/ccmake /usr/bin/ccmake
-RUN cd /opt && unzip /tmp/android-ndk-r17b-linux-x86_64.zip
-ENV NDK_ROOT /opt/android-ndk-r17b
+RUN cd /opt && unzip /tmp/android-ndk-r17c-linux-x86_64.zip
+ENV NDK_ROOT /opt/android-ndk-r17c