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.

.github/workflows/build.yml

@@ -65,24 +65,20 @@ jobs:
         name: Mac flatc binary
         path: Release/flatc
 
-
-# FIXME: to make this work, this apparently needs:
-# - a newer version of gradle in build.gradle (e.g. 3.2.1)
-# - google() in the repos in build.gradle
-# - gradle version 4.6 in gradle-wrapper.properties ?
-# - some kind of flavor declaration?
-# All while not breaking the existing Travis Android build.
-# good luck.
-
-#  build-android:
-#    name: Build Android (on Linux)
-#    runs-on: ubuntu-latest
-#    steps:
-#    - uses: actions/checkout@v1
-#    - name: set up JDK 1.8
-#      uses: actions/setup-java@v1
-#      with:
-#        java-version: 1.8
-#    - name: build
-#      working-directory: android
-#      run: bash ./gradlew buildDebug
+  build-android:
+   name: Build Android (on Linux)
+   runs-on: ubuntu-latest
+   steps:
+   - uses: actions/checkout@v1
+   - name: set up JDK 1.8
+     uses: actions/setup-java@v1
+     with:
+       java-version: 1.8
+   - name: set up flatc
+     run: |
+       cmake -DFLATBUFFERS_BUILD_TESTS=OFF -DFLATBUFFERS_BUILD_FLATLIB=OFF -DFLATBUFFERS_BUILD_FLATHASH=OFF .
+       make
+       echo "${PWD}" >> $GITHUB_PATH
+   - name: build
+     working-directory: android
+     run: bash ./gradlew clean build