Configurable start index and max length for Emoji application

I tested it a bit, it should work fine. I made this feature available only to TextViews.

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>
How about this:

```
int textLength = text.length();
int textLengthToProcessMax = textLength - index;
int textLengthToProcess = length < 0 || length >= textLengthToProcessMax ? textLengthToProcessMax : length;

// then use textLengthToProcess instead of length. It's a good practice to not modify any method parameters.
```

I haven't tested the code above yet. But hopefully you get the idea.