Custom user-agent option

port from https://github.com/CyanogenMod/android_packages_apps_Mms/commit/afac2716ff7c43234fe1ba4a0247a25707c3a6c1
Custom user-agent is necessary for MMS access on the Softbank mobile network in Japan.

PS2: rebase
PA3: rebase

Change-Id: Ia3743360cb4660babbfa3311de016ee3b888a858
Signed-off-by: rdgate <rdgate@gmail.com>