Store phone blacklist in content provider (2/4)

Change-Id: I0fe685ff0a63848d666fc76510e4a331b74b94fe
telephony: Add support for choosing a default APN

 * Sometimes, the standard method of choosing the "first" available APN
   is not what we want. Consider the case where a certain provider has
   both a "normal" APN and an LTE APN. The LTE APN is what we want, but
   it's not chosen.
 * Add a new overlayable value "config_preferred_apn" to handle this. A
   device can override this. The format is "apn,mcc,mnc".

Change-Id: Ia6ca4159491bee15b3f18ad7ad524b8b0ffce2f2
Improve delete handling code.

Change-Id: I3f09f00a2966ee246056ea11ec55d952da6aabc0
Drop blacklist number uniqueness constraint.

It's very hard to deal with that constraint in practise, so drop the
contraint from the DB and handle potential duplicate rows when querying.

Change-Id: Id28a415ba80cf100879b24de88b4b50638e900ca