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.

android/build.gradle

@@ -1,108 +1,35 @@
-// Copyright (c) 2017 Google, Inc.
-//
-// This software is provided 'as-is', without any express or implied
-// warranty.  In no event will the authors be held liable for any damages
-// arising from the use of this software.
-// Permission is granted to anyone to use this software for any purpose,
-// including commercial applications, and to alter it and redistribute it
-// freely, subject to the following restrictions:
-// 1. The origin of this software must not be misrepresented; you must not
-// claim that you wrote the original software. If you use this software
-// in a product, an acknowledgment in the product documentation would be
-// appreciated but is not required.
-// 2. Altered source versions must be plainly marked as such, and must not be
-// misrepresented as being the original software.
-// 3. This notice may not be removed or altered from any source distribution.
-
+// Top-level build file where you can add configuration options common to all sub-projects/modules.
 buildscript {
+  ext.kotlin_version = "1.4.10"
   repositories {
+    google()
     jcenter()
   }
   dependencies {
-    classpath 'com.android.tools.build:gradle:2.3.0'
+    classpath 'com.android.tools.build:gradle:4.1.0'
+    classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
+
+    // NOTE: Do not place your application dependencies here; they belong
+    // in the individual module build.gradle files
   }
 }
 
 allprojects {
   repositories {
+    google()
     jcenter()
   }
 }
 
-apply plugin: 'com.android.application'
+tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).all {
+  sourceCompatibility = JavaVersion.VERSION_1_8
+  targetCompatibility = JavaVersion.VERSION_1_8
 
-android {
-  compileSdkVersion 25
-  buildToolsVersion '25.0.2'
-
-  sourceSets {
-    main {
-      manifest.srcFile 'AndroidManifest.xml'
-      res.srcDirs = ['res']
-    }
-  }
-
-  externalNativeBuild {
-    ndkBuild {
-      path "jni/Android.mk"
-    }
-  }
-
-  defaultConfig {
-    applicationId 'com.example.FlatBufferTest'
-    // This is the platform API where NativeActivity was introduced.
-    minSdkVersion 9
-    targetSdkVersion 25
-    versionCode 1
-    versionName "1.0"
-
-    buildTypes {
-      release {
-        minifyEnabled false
-      }
-    }
-
-    externalNativeBuild {
-      ndkBuild {
-        targets "FlatBufferTest"
-        arguments "-j" + Runtime.getRuntime().availableProcessors()
-        abiFilters "armeabi-v7a", "arm64-v8a", "x86", "x86_64"
-      }
-    }
-  }
-
-  lintOptions {
-    abortOnError false
-  }
-
-  // Build with each STL variant.
-  productFlavors {
-    stlport {
-      applicationIdSuffix ".stlport"
-      versionNameSuffix "-stlport"
-      externalNativeBuild {
-        ndkBuild {
-          arguments "APP_STL=stlport_static"
-        }
-      }
-    }
-    gnustl {
-      applicationIdSuffix ".gnustl"
-      versionNameSuffix "-gnustl"
-      externalNativeBuild {
-        ndkBuild {
-          arguments "APP_STL=gnustl_static"
-        }
-      }
-    }
-    libcpp {
-      applicationIdSuffix ".libcpp"
-      versionNameSuffix "-libcpp"
-      externalNativeBuild {
-        ndkBuild {
-          arguments "APP_STL=c++_static"
-        }
-      }
-    }
+  compileKotlin {
+    dependsOn flatbuffer
   }
 }
+
+task clean(type: Delete) {
+  delete rootProject.buildDir
+}