Gradle (#649)

* Gradle

Gradle support to make ease of import in Android Java projects.

* Gradle

Gradle support for Android projects.
For command line build, `gradle assembe` will trigger build phase

 - Config for Android .so file generation
 - Commented build.gradle
 - Automatic file copy after build
 - Dummy AndroidManifest file

* Build with Gradle

 - Build job for Travis CI
 - Moved gradle file to support/

* Gradle Path

 - Absolute path for Gradle binary

* File check after Gradle build

 - Additional script for build success
 - Check the Gradle's `assemble.doLast` task

.travis.yml

@@ -21,6 +21,36 @@ matrix:
   exclude:
     - os: osx
       env: BUILD=Doc
+  include:
+    - language: android
+      android:
+      components:
+        - tools
+        - platform-tools
+        - android-21
+        - sys-img-armeabi-v7a-android-21
+      env:
+        - ANDROID=true
+      before_install:
+        - git submodule update --init --recursive
+        - sudo apt-get install wget unzip tree
+      install:
+        # Accept SDK Licenses + Install NDK
+        - yes | sdkmanager --update > /dev/null 2>&1
+        - sdkmanager ndk-bundle > /dev/null 2>&1
+        # Download Gradle 4.3.1 
+        - wget https://services.gradle.org/distributions/gradle-4.3.1-bin.zip
+        - mkdir -p gradle
+        - unzip -q -d ./gradle gradle-4.3.1-bin.zip
+        - export GRADLE=${TRAVIS_BUILD_DIR}/gradle/gradle-4.3.1/bin/gradle
+      before_script:
+        - bash $GRADLE --version
+        - cd ./support
+      script:
+        - bash $GRADLE clean assemble
+      after_success:
+        - cd ${TRAVIS_BUILD_DIR}
+        - tree ./libs
 
 # Install gcc-6 for extended constexpr support.
 addons: