[app] Introduce CheckoutApplication and prepare Billing

As there should be only one instance of Billing in the app it is
instantiated in the Application class. It can later be used via
`Activity#getApplication().getBilling()`.

Encryption class is added in this commit in order to encrypt/decrypt
the key.

Author: serso

app/src/main/AndroidManifest.xml

@@ -22,6 +22,7 @@
 
 <manifest
     xmlns:android="http://schemas.android.com/apk/res/android"
+    xmlns:tools="http://schemas.android.com/tools"
     package="org.solovyev.android.checkout.app">
 
     <supports-screens android:smallScreens="true" />
@@ -32,4 +33,12 @@
 
     <uses-permission android:name="com.android.vending.BILLING" />
 
+    <application
+        android:name=".CheckoutApplication"
+        android:allowBackup="true"
+        android:icon="@drawable/ic_launcher"
+        android:label="@string/app_name"
+        android:supportsRtl="true"
+        tools:ignore="GoogleAppIndexingWarning" />
+
 </manifest>

app/src/main/java/org/solovyev/android/checkout/app/CheckoutApplication.java

@@ -0,0 +1,34 @@
+package org.solovyev.android.checkout.app;
+
+import org.solovyev.android.checkout.Billing;
+
+import android.app.Application;
+
+import javax.annotation.Nonnull;
+
+public class CheckoutApplication extends Application {
+
+    @Nonnull
+    private final Billing mBilling = new Billing(this, new Billing.DefaultConfiguration() {
+        @Nonnull
+        @Override
+        public String getPublicKey() {
+            // encrypted public key of the app. Plain version can be found in Google Play's Developer
+            // Console in Service & APIs section under "YOUR LICENSE KEY FOR THIS APPLICATION" title.
+            // A naive encryption algorithm is used to "protect" the key. See more about key protection
+            // here: https://developer.android.com/google/play/billing/billing_best_practices.html#key
+            final String s = "PixnMSYGLjg7Ah0xDwYILlVZUy0sIiBoMi4jLDcoXTcNLiQjKgtlIC48NiRcHxwKHEcYEyZrPyMWXFRpV10VES9ENz" +
+                    "g1Hj06HTV1MCAHJlpgEDcmOxFDEkA8OiQRKjEQDxhRWVVEMBYmNl1AJghcKUAYVT15KSQgBQABMgwqKSlqF1gZBA4fAw5rMyxKI" +
+                    "w9LJFc7AhxZGjoPATgRUiUjKSsOWyRKDi4nIA9lKgAGOhMLDF06CwoKGFR6Wj0hGwReS10NXzQTIREhKlkuMz4XDTwUQjRCJUA+" +
+                    "VjQVPUIoPicOLQJCLxs8RjZnJxY1OQNLKgQCPj83AyBEFSAJEk5UClYjGxVLNBU3FS4DCztENQMuOk5rFVclKz88AAApPgADGFx" +
+                    "EEV5eQAF7QBhdQEE+Bzc5MygCAwlEFzclKRB7FB0uFgwPKgAvLCk2OyFiKxkgIy8BBQYjFy4/E1ktJikrEVlKJVYIHh16NDwtDC" +
+                    "U0Vg8JNzoQBwQWOwk1GzZ4FT8fWicwITcRJi8=";
+            return Encryption.decrypt(s, "se.solovyev@gmail.com");
+        }
+    });
+
+    @Nonnull
+    public Billing getBilling() {
+        return mBilling;
+    }
+}

app/src/main/java/org/solovyev/android/checkout/app/Encryption.java

@@ -0,0 +1,40 @@
+package org.solovyev.android.checkout.app;
+
+import android.util.Base64;
+
+import javax.annotation.Nonnull;
+
+/**
+ * Simple (and stupid) encryption algorithm. Should not be used in real apps.
+ */
+final class Encryption {
+
+    @Nonnull
+    static String decrypt(@Nonnull String message, @Nonnull String salt) {
+        return xor(new String(Base64.decode(message, 0)), salt);
+    }
+
+    @Nonnull
+    static String encrypt(@Nonnull String message, @Nonnull String salt) {
+        return new String(Base64.encode(xor(message, salt).getBytes(), 0));
+    }
+
+    /**
+     * Encrypts or decrypts a base-64 string using a XOR cipher.
+     */
+    @Nonnull
+    private static String xor(@Nonnull String message, @Nonnull String salt) {
+        final char[] m = message.toCharArray();
+        final int ml = m.length;
+
+        final char[] s = salt.toCharArray();
+        final int sl = s.length;
+
+        final char[] res = new char[ml];
+        for (int i = 0; i < ml; i++) {
+            res[i] = (char) (m[i] ^ s[i % sl]);
+        }
+        return new String(res);
+    }
+
+}

app/src/main/res/drawable-hdpi/ic_launcher.png

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+    <string name="app_name">Checkout</string>
+</resources>

app/src/main/res/drawable-mdpi/ic_launcher.png

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="utf-8"?>
+<resources>
+    <string name="app_name">Checkout</string>
+</resources>