Use the same HabitCardListCache for all activities

Fixes #147

app/src/main/java/org/isoron/uhabits/AppComponent.java

@@ -19,6 +19,7 @@
 
 package org.isoron.uhabits;
 
+import org.isoron.uhabits.activities.habits.list.model.*;
 import org.isoron.uhabits.commands.*;
 import org.isoron.uhabits.intents.*;
 import org.isoron.uhabits.io.*;
@@ -70,4 +71,6 @@ public interface AppComponent
     WidgetPreferences getWidgetPreferences();
 
     WidgetUpdater getWidgetUpdater();
+
+    HabitCardListCache getHabitCardListCache();
 }

app/src/main/java/org/isoron/uhabits/activities/habits/list/model/HabitCardListCache.java

@@ -21,7 +21,6 @@ package org.isoron.uhabits.activities.habits.list.model;
 
 import android.support.annotation.*;
 
-import org.isoron.uhabits.activities.*;
 import org.isoron.uhabits.commands.*;
 import org.isoron.uhabits.models.*;
 import org.isoron.uhabits.tasks.*;
@@ -38,8 +37,11 @@ import javax.inject.*;
  * This is needed since performing database lookups during scrolling can make
  * the ListView very slow. It also registers itself as an observer of the
  * models, in order to update itself automatically.
+ * <p>
+ * Note that this class is singleton-scoped, therefore it is shared among all
+ * activities.
  */
-@ActivityScope
+@Singleton
 public class HabitCardListCache implements CommandRunner.Listener
 {
     private int checkmarkCount;