Migrating MmsConfig to CarrierConfigManager.

Define variables in CarrierConfigManager that mirror those in
SmsManager. These will replace MmsConfig. The literal values had to be
moved into this class because classes in frameworks/base/telephony
cannot reference classes in frameworks/opt/telephony.

There is no change to the SmsManager API, but the data available from
SmsManager#getCarrierConfigValues is moving from mms_config.xml files
into the new carrier config API.

Bug: 21085630
Change-Id: I8a88f46c770ddcb3ac6988c29d86f0ca1f211f4a