Added android build script

tool/android.sh

@@ -0,0 +1,22 @@
+#!/bin/bash
+
+if ! [ -e "pubspec.yaml" ] || ! [ -d "oxen_coin" ]; then
+    echo "Error: this script must be invoked from the top-level project directory" >&2
+    exit 1
+fi
+
+set -e -x
+
+./tool/download-android-deps.sh https://oxen.rocks/oxen-io/oxen-core/oxen-stable-android-deps-LATEST.tar.xz
+
+flutter pub get
+
+flutter pub upgrade
+
+flutter pub run build_runner build --delete-conflicting-outputs
+
+flutter pub run flutter_launcher_icons:main
+
+flutter build apk --split-per-abi
+
+flutter build appbundle