Fix emoji backwards compatibility recents crash

```
java.lang.NumberFormatException: unable to parse '1f601.png' as integer
    at java.lang.Integer.parse(Integer.java:383)
    at java.lang.Integer.parseInt(Integer.java:372)
    at org.thoughtcrime.securesms.util.Emoji.getRecentlyUsed(Emoji.java:267)
    at org.thoughtcrime.securesms.components.EmojiDrawer$EmojiGridAdapter.getView(EmojiDrawer.java:226)
```

The old drawer stored recent emojis with a .png suffix. There's a bunch of places/ways this could be fixed in, here's my suggestion.