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

Conflicts:
	src/com/android/providers/telephony/TelephonyProvider.java

Conflicts:
	src/com/android/providers/telephony/TelephonyProvider.java
TelephonyProvider: allow overriding default apn via a prop

Consolidated builds cannot specify variant-specific stuff via overlays
anymore. Allow setting this using a prop.

Change-Id: I656196f1ec76c382ecb95a2c1d9f8eec961eddd6