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/app/src/main/java/com/flatbuffers/app/MainActivity.kt

@@ -0,0 +1,51 @@
+package com.flatbuffers.app
+
+import android.annotation.SuppressLint
+import androidx.appcompat.app.AppCompatActivity
+import android.os.Bundle
+import android.widget.TextView
+import com.fbs.app.Animal
+import com.google.flatbuffers.FlatBufferBuilder
+import java.nio.ByteBuffer
+
+@ExperimentalUnsignedTypes
+class MainActivity : AppCompatActivity() {
+
+  @SuppressLint("SetTextI18n")
+  override fun onCreate(savedInstanceState: Bundle?) {
+    super.onCreate(savedInstanceState)
+    setContentView(R.layout.activity_main)
+
+    val tiger = Animal.getRootAsAnimal(ByteBuffer.wrap(createAnimalFromJNI()))
+    findViewById<TextView>(R.id.tv_animal_one).text = animalInfo(tiger)
+
+    findViewById<TextView>(R.id.tv_animal_two).text = animalInfo(createAnimalFromKotlin())
+  }
+
+  // This function is a sample of communicating FlatBuffers between JNI (native C++) and Java.
+  // Implementation can be found on animals.cpp file.
+  private external fun createAnimalFromJNI(): ByteArray
+
+  // Create a "Cow" Animal flatbuffers from Kotlin
+  private fun createAnimalFromKotlin():Animal {
+    val fb = FlatBufferBuilder(100)
+    val cowOffset = Animal.createAnimal(
+      builder = fb,
+      nameOffset = fb.createString("Cow"),
+      soundOffset = fb.createString("Moo"),
+      weight = 720u
+    )
+    fb.finish(cowOffset)
+    return Animal.getRootAsAnimal(fb.dataBuffer())
+  }
+
+  private fun animalInfo(animal: Animal): String =
+    "The ${animal.name} sound is ${animal.sound} and it weights ${animal.weight}kg."
+
+  companion object {
+    // Used to load the 'native-lib' library on application startup.
+    init {
+      System.loadLibrary("native-lib")
+    }
+  }
+}

android/app/src/main/java/generated/com/fbs/app/Animal.kt

@@ -0,0 +1,64 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+
+package com.fbs.app
+
+import java.nio.*
+import kotlin.math.sign
+import com.google.flatbuffers.*
+
+@Suppress("unused")
+@ExperimentalUnsignedTypes
+class Animal : Table() {
+
+    fun __init(_i: Int, _bb: ByteBuffer)  {
+        __reset(_i, _bb)
+    }
+    fun __assign(_i: Int, _bb: ByteBuffer) : Animal {
+        __init(_i, _bb)
+        return this
+    }
+    val name : String?
+        get() {
+            val o = __offset(4)
+            return if (o != 0) __string(o + bb_pos) else null
+        }
+    val nameAsByteBuffer : ByteBuffer get() = __vector_as_bytebuffer(4, 1)
+    fun nameInByteBuffer(_bb: ByteBuffer) : ByteBuffer = __vector_in_bytebuffer(_bb, 4, 1)
+    val sound : String?
+        get() {
+            val o = __offset(6)
+            return if (o != 0) __string(o + bb_pos) else null
+        }
+    val soundAsByteBuffer : ByteBuffer get() = __vector_as_bytebuffer(6, 1)
+    fun soundInByteBuffer(_bb: ByteBuffer) : ByteBuffer = __vector_in_bytebuffer(_bb, 6, 1)
+    val weight : UShort
+        get() {
+            val o = __offset(8)
+            return if(o != 0) bb.getShort(o + bb_pos).toUShort() else 0u
+        }
+    companion object {
+        fun validateVersion() = Constants.FLATBUFFERS_1_12_0()
+        fun getRootAsAnimal(_bb: ByteBuffer): Animal = getRootAsAnimal(_bb, Animal())
+        fun getRootAsAnimal(_bb: ByteBuffer, obj: Animal): Animal {
+            _bb.order(ByteOrder.LITTLE_ENDIAN)
+            return (obj.__assign(_bb.getInt(_bb.position()) + _bb.position(), _bb))
+        }
+        fun createAnimal(builder: FlatBufferBuilder, nameOffset: Int, soundOffset: Int, weight: UShort) : Int {
+            builder.startTable(3)
+            addSound(builder, soundOffset)
+            addName(builder, nameOffset)
+            addWeight(builder, weight)
+            return endAnimal(builder)
+        }
+        fun startAnimal(builder: FlatBufferBuilder) = builder.startTable(3)
+        fun addName(builder: FlatBufferBuilder, name: Int) = builder.addOffset(0, name, 0)
+        fun addSound(builder: FlatBufferBuilder, sound: Int) = builder.addOffset(1, sound, 0)
+        fun addWeight(builder: FlatBufferBuilder, weight: UShort) = builder.addShort(2, weight.toShort(), 0)
+        fun endAnimal(builder: FlatBufferBuilder) : Int {
+            val o = builder.endTable()
+            return o
+        }
+        fun finishAnimalBuffer(builder: FlatBufferBuilder, offset: Int) = builder.finish(offset)
+        fun finishSizePrefixedAnimalBuffer(builder: FlatBufferBuilder, offset: Int) = builder.finishSizePrefixed(offset)
+    }
+}