[app] Allow get CheckoutApplication from any Activity

Author: serso

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

@@ -2,6 +2,7 @@
 
 import org.solovyev.android.checkout.Billing;
 
+import android.app.Activity;
 import android.app.Application;
 
 import javax.annotation.Nonnull;
@@ -27,6 +28,13 @@ public String getPublicKey() {
         }
     });
 
+    /**
+     * Returns an instance of {@link CheckoutApplication} attached to the passed activity.
+     */
+    public static CheckoutApplication get(Activity activity) {
+        return (CheckoutApplication) activity.getApplication();
+    }
+
     @Nonnull
     public Billing getBilling() {
         return mBilling;