app_record: Don't set RECORD_STATUS chan var until file is closed

We've been calling pbx_builtin_setvar_helper to set the
RECORD_STATUS variable before actually closing the recorded file.
If a client is watching VarSet events and tries to do something with
the file when a RECORD_STATUS event is seen, they might attempt to
do so while the file it's still open.

We now delay calling pbx_builtin_setvar_helper until after we close
the file.

ASTERISK-27423

Change-Id: I7fe9de99953e46b4bafa2b38cf151fe8f6488254