fix recent emoji pane

1) Make recent list properly update and invalidate.
2) Show most-recently-used first.
3) Refactoring

Closes #3171
// FREEBIE

src/org/thoughtcrime/securesms/components/emoji/StaticEmojiPageModel.java

@@ -3,7 +3,7 @@ package org.thoughtcrime.securesms.components.emoji;
 import android.support.annotation.DrawableRes;
 import android.support.annotation.NonNull;
 
-public class StaticEmojiPageModel implements EmojiPageModel {
+public class StaticEmojiPageModel extends EmojiPageModel {
   @DrawableRes private final int   icon;
   @NonNull     private final int[] codePoints;
 
@@ -19,6 +19,4 @@ public class StaticEmojiPageModel implements EmojiPageModel {
   @NonNull public int[] getCodePoints() {
     return codePoints;
   }
-
-  @Override public void onCodePointSelected(int codePoint) { }
 }