keyboard: Use the right property to figure out modifiers set size
During development of the OSK updates, this._modifiers turned from an array into a Set(), but this location missed switching from length() to size() to get the cardinality of the contained values.
As a result, the OSK misbehaved and resorted for key event forwarding for impossible keyvals, instead of handing them directly to the IM.
Fixes: #5766 (closed)