Read MMS config from CarrierConfigManager.

This adds a helper method for use by MmsConfigManager, so that it can
strip out miscellaneous carrier config variables from
CarrierConfigManager leaving only MMS config.  There should be no
visible change in the public SmsManager API, which still exposes the
same data in getCarrierConfigValues.

Bug: 21085630
Change-Id: I2eb6df70d5ec5a61584ec35dd0b5d99c57be3ca4