gsm: Show localized SPN only when it matches system locale (1/2)

This adds child tag <spn locale="LOCALE">SPN</spn> to <spnOverride/> of
SPN overrides list (/system/etc/spn-conf.xml). LOCALE takes the format
of java.util.Locale (e.g.: en en_US). For example of the xml file, see
the new revision in part 2 - http://review.cyanogenmod.com/10038

SPN override used to be assigned only on SIM ready. Now it is updated on
every system locale change. It's applied when:

* There is a <spn> entry of the carrier whose locale= attribute matches
  the current system locale.

* No <spn> entry of the carrier that matches, but <spnOverride>'s spn=
  attribute (as the default SPN) is provided.

Otherwise, meaning no locale= matches and spn= is also omitted, the SPN
that read from SIM card would be applied.

Change-Id: Ib488219f8e8d8cd0962136d0a581c88ecb8458c4