MMS: Fix Screen is not smooth when scroll to read long MMS

When scroll the mobile paper show screen the ScrollView is
scrolled by the scrollby method and did not call the super
method.

call back the same super method,and remove the scrollby
method make the ScrollView to scroll quickly.

CRs-Fixed: 732278

Change-Id: I1c530fc4569b93b22afe68c2106244959efde65d