Add a default MMS UA

The default user-agent isn't set, which makes
TelephonyManager.getMmsUserAgent() return an empty string when a
device doesn't have one in its own overlays. Every hardware should
set its own unique name for this, but add a default value for cases
in which is is forgotten.

Fixes android.telephony.cts.TelephonyManagerTest#testTelephonyManager

Change-Id: I8a648014873319ea0d0f1089495d6777b8a1b8bf