Migrating MmsConfig to CarrierConfigManager.

This brings MMS_CONFIG_SUPPORT_HTTP_CHARSET_HEADER from MmsConfig.java
into the public API.

Because classes in frameworks/base/telephony cannot refer to classes in
frameworks/opt/telephony, the literal values of these constants are
moving to CarrierConfigManager. The public API is not affected by this
shuffling.

Bug: 21085630
Change-Id: I2e6897890e182fb181a6d8453c3f1780cb323045