Add Custom MMS User-Agent configuration option

More than few carriers restrict MMS messages on their nework by filtering MMS client User-Agent strings. Some notorious carriers even preven MMS from being sent if the UA sting does not match their own obscure list.
This add additional settings option, User-Agent override, with the ability to select few popular User-Agent strings, as well as typing in a custom information.

Change-Id: I5a68827039c815f159773c4aad96063a0d98d9e4

Conflicts:
	res/values/arrays.xml
	res/xml/preferences.xml
MMS: Fixed using checkstyle

Change-Id: I71ec8a5bdb7749277d3915e78f54edba079915a0