Use "sms_body" extra from "smsto:" Intent

When processing a SENDTO intent, the stock SMS app also populates the text of the draft from the `sms_body` extra (if present), while TextSecure does not.

The fix is trivial and should pose no risks, and it brings TextSecure's behaviour more in line with the stock app.

`Intent.getStringExtra` is documented to return `null` in case there's no extra of the requested name, so no error checking is necessary.
Why do this here and not with the declaration?