If no text is selected, this is the same as passing the caret offset, since the selection range is already where we want it.
However, if text is selected, this allows the caller to replace it, just as it would be when pasting using the keyboard or GUI.
Note that this doesn't break any existing expectations because TEXT_OFFSET_CARET was never supported for this method previously.
This allows us to remove the DeleteText call from Android SessionAccessibility::Paste.
This separate DeleteText call was causing problems because it could result in the deletion and the insertion happening in two separate ticks, resulting in incorrect events.
Now, this should always happen in a single tick.
Differential Revision: https://phabricator.services.mozilla.com/D226108
