Telephony: bugfix - let us edit 'My phone number' again

http://review.cyanogenmod.org/#/c/75688/3 introduced a change on "My Phone
Number", having the option disabled at the beginning, so it can only be used
after it has been init'd.
This commit now enables the option, when init is done.

Change-Id: I50438ac5231934b9c28d7161c05438ab4519aa8b