apn: Fixup legacy 'read_only' attribute

Replace with 'user_editable="false"', because 'read_only' hasn't been
around anymore for a long time.

Networks:
* 234,30 and 234,38 - Virgin UK
* 730,03 - Claro

Issue: FP3-A11#360
Change-Id: I21bc57c7f9e3cee9bd87125527d15913dd69c091
1 file changed