Cache Flag and GallerySetting state

The loading of GallerySettings has a high latency in production. This
commit makes it so that the GallerySetting object is created only once
per GAE instance. It also updates the Flag class to cache the value on
first read since these flags are set in appengine-web.xml and are only
expected to change during deployment of a new version of App Inventor.

Change-Id: I7bb123697e821f2510807c5cbd5671bae71db307

appinventor/appengine/src/com/google/appinventor/server/GalleryServiceImpl.java

@@ -35,7 +35,6 @@ import com.google.appinventor.server.storage.GalleryStorageIoInstanceHolder;
 import com.google.appinventor.shared.rpc.project.Email;
 import com.google.appinventor.shared.rpc.project.GalleryApp;
 import com.google.appinventor.shared.rpc.project.GalleryAppListResult;
-import com.google.appinventor.shared.rpc.project.GalleryAppReport;
 import com.google.appinventor.shared.rpc.project.GalleryComment;
 import com.google.appinventor.shared.rpc.project.GalleryModerationAction;
 import com.google.appinventor.shared.rpc.project.GalleryReportListResult;
@@ -59,14 +58,18 @@ public class GalleryServiceImpl extends OdeRemoteServiceServlet implements Galle
       GalleryStorageIoInstanceHolder.INSTANCE;
   // fileExporter used to get the source code from project being published
   private final FileExporter fileExporter = new FileExporterImpl();
+  private final GallerySettings settings;
 
-  @Override
-  public GallerySettings loadGallerySettings() {
+  public GalleryServiceImpl() {
     String bucket = Flag.createFlag("gallery.bucket", "").get();
     boolean galleryEnabled = Flag.createFlag("use.gallery",false).get();
     String envirnment = SystemProperty.environment.value().toString();
     String adminEmail = Flag.createFlag("gallery.admin.email", "").get();
-    GallerySettings settings = new GallerySettings(galleryEnabled, bucket, envirnment, adminEmail);
+    settings = new GallerySettings(galleryEnabled, bucket, envirnment, adminEmail);
+  }
+
+  @Override
+  public GallerySettings loadGallerySettings() {
     return settings;
   }
 

appinventor/appengine/src/com/google/appinventor/server/flags/Flag.java

@@ -18,6 +18,7 @@ package com.google.appinventor.server.flags;
 public abstract class Flag<T> {
   private final String name;
   private final T defaultValue;
+  private T cachedValue = null;
 
   protected Flag(String name, T defaultValue) {
     this.name = name;
@@ -30,8 +31,11 @@ public abstract class Flag<T> {
    * @return the current value of this flag.
    */
   public final T get() throws IllegalFlagValueException {
-    String value = System.getProperty(name);
-    return (value == null) ? defaultValue : convert(value);
+    if (cachedValue == null) {
+      String value = System.getProperty(name);
+      cachedValue = (value == null) ? defaultValue : convert(value);
+    }
+    return cachedValue;
   }
 
   /**
@@ -41,6 +45,7 @@ public abstract class Flag<T> {
    */
   public final void setForTest(T value) {
     System.setProperty(name, value.toString());
+    cachedValue = null;
   }
 
   /**