Feature/craig/ensure ndk installed during build (#784)

* Ensure NDK version installed

* Update license agreements

Author: CDRussell

install_tools.sh

@@ -1,6 +1,9 @@
 mkdir "$ANDROID_HOME/licenses" || true;
 echo "8933bad161af4178b1185d1a37fbf41ea5269c55" > "$ANDROID_HOME/licenses/android-sdk-license";
 echo "d56f5187479451eabf01fb78af6dfcb131a6481e" > "$ANDROID_HOME/licenses/android-sdk-license";
+echo "d56f5187479451eabf01fb78af6dfcb131a6481e" > "$ANDROID_HOME/licenses/android-sdk-license";
+echo "24333f8a63b6825ea9c5514f83c2829b004d1fee" > "$ANDROID_HOME/licenses/android-sdk-license";
+sdkmanager 'ndk;21.0.6113669'
 sdkmanager tools;
 yes | sdkmanager --licenses;
 echo "Fetching ndk-bundle. Suppressing output to avoid travis 4MG size limit";