Pin FlatBuffers Android Travis builds to NDK r17c (#5460)

r17c is the last Android NDK to include stlport and gnustl.
We want to continue to support these deprecated STLs until we have
confidence few enough customers are using them.

.travis.yml

@@ -180,6 +180,7 @@ matrix:
 
     - language: android
       sudo: true
+      dist: trusty
       android:
         components:
           - tools
@@ -193,8 +194,20 @@ matrix:
       before_install:
         # Output something every 10 minutes or Travis kills the job
         - while sleep 540; do echo "=====[ $SECONDS seconds still running ]====="; done &
-        - git clone https://github.com/urho3d/android-ndk.git $HOME/android-ndk-root
-        - export ANDROID_NDK_HOME=$HOME/android-ndk-root
+        # Install the r17c version of the NDK that still so that we can continue to test with gnustl
+        # and stlport.
+        - export ANDROID_NDK_HOME=$HOME/android-ndk
+        - NDK_ZIP=$ANDROID_NDK_HOME/ndk.zip
+        - mkdir -p $ANDROID_NDK_HOME
+        - curl -o $NDK_ZIP https://dl.google.com/android/repository/android-ndk-r17c-linux-x86_64.zip
+        - unzip -q -d $ANDROID_NDK_HOME $NDK_ZIP
+        - rm $NDK_ZIP
+        - mv $ANDROID_NDK_HOME/android-ndk-*/* $ANDROID_NDK_HOME
+        - rmdir $ANDROID_NDK_HOME/android-ndk-*
+        - export CMAKE=$(which cmake)
+        # libc required for prebuilt llvm toolchain the NDK r17c.
+        - if [ "$TRAVIS_OS_NAME" == "linux" ]; then sudo apt-get update -qq; fi
+        - if [ "$TRAVIS_OS_NAME" == "linux" ]; then sudo apt-get install -qq libc6; fi
         # Setup environment for Linux build which is required to build the sample.
         - if [ "$TRAVIS_OS_NAME" == "linux" ]; then sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test; fi
         - if [ "$TRAVIS_OS_NAME" == "linux" ]; then sudo apt-get update -qq; fi