Add "Roaming protocol" to APN editor.

Field already exists in the APN database.  Now it is user updatable.
Only enable it on CDMA for this release (means it is not really user
visible, but available for testing purposes).  Will revisit enabling
for all in a future release.

Bug: 5420680
Change-Id: Ia2c551c48ae9ce47c57ad36ec73d152441a27284