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/res/values/colors.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+    <color name="colorPrimary">#6200EE</color>
+    <color name="colorPrimaryDark">#3700B3</color>
+    <color name="colorAccent">#03DAC5</color>
+</resources>

android/app/src/main/res/values/strings.xml

@@ -0,0 +1,3 @@
+<resources>
+  <string name="app_name">FlatbuffersTestApp</string>
+</resources>

android/app/src/main/res/values/styles.xml

@@ -0,0 +1,10 @@
+<resources>
+    <!-- Base application theme. -->
+    <style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
+        <!-- Customize your theme here. -->
+        <item name="colorPrimary">@color/colorPrimary</item>
+        <item name="colorPrimaryDark">@color/colorPrimaryDark</item>
+        <item name="colorAccent">@color/colorAccent</item>
+    </style>
+
+</resources>