Modernize android build and sample (#6229)

Android build was dated, using the Android.mk approach. Current
project configuration on Android encourages the usage of CMake, so we
are updating the android project as an example on how to use either the
Java/Kotlin generate code or the native C++ one.

.travis.yml

@@ -185,37 +185,18 @@ matrix:
         components:
           - tools
           - platform-tools
-          - build-tools-25.0.2
-          - android-25
           - extra-android-m2repository
+        licenses:
+          - 'android-sdk-preview-license-52d11cd2'
+          - 'android-sdk-license-.+'
+          - 'google-gdk-license-.+'
       compiler:
         - gcc
-
       before_install:
-        # Output something every 10 minutes or Travis kills the job
-        - while sleep 540; do echo "=====[ $SECONDS seconds still running ]====="; done &
-        # Install the r17c version of the NDK that still so that we can continue to test with gnustl
-        # and stlport.
-        - export ANDROID_NDK_HOME=$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
-        - if [ "$TRAVIS_OS_NAME" == "linux" ]; then sudo apt-get install -qq g++-$GCC_VERSION; fi
-        - if [ "$TRAVIS_OS_NAME" == "linux" ]; then sudo apt-get install -qq gcc-$GCC_VERSION; fi
-        - if [ "$TRAVIS_OS_NAME" == "linux" ]; then sudo ln -s -v -f $(which g++-$GCC_VERSION) /usr/bin/g++; fi
-        - if [ "$TRAVIS_OS_NAME" == "linux" ]; then sudo ln -s -v -f $(which gcc-$GCC_VERSION) /usr/bin/gcc; fi
+        - echo y | sdkmanager "platforms;android-30"
+        - echo y | sdkmanager "build-tools;30.0.2"
+        - echo y | sdkmanager "ndk-bundle"
+        - echo y | sdkmanager "cmake;3.6.4111459"
       script:
-        - failed=0; for build_gradle in $(git ls-files | grep build.gradle); do ( cd "$(dirname "${build_gradle}")" && ./gradlew build ) || failed=1; done; exit $((failed))
-        # Kill the sleep loop
-        - kill %1
+        - cmake -DFLATBUFFERS_BUILD_TESTS=OFF -DFLATBUFFERS_BUILD_FLATLIB=OFF -DFLATBUFFERS_BUILD_FLATHASH=OFF .; make; export PATH="$PATH:${PWD}"
+        - cd android; ./gradlew clean build