Adding sharedPref for blocked number migration

+ Users are shown a dialog when they're running on an SDK which
supports the framework blocking solution, but they haven't yet
migrated. In order to determine whether the user has migrated or not,
a SharedPreference value is used. In a later CL which performs the
migration, this value will be updated as the final step.

Bug: 26664600
Change-Id: I5a12be643d0fb3b52ef408215779423bf0a2ddc7