Checking for blocked number after migrating

+ There's an edge case crash in the Dialer when initiating the
migration workflow. If the user has blocked a number in the framework
prior to migrating the Dialer and then attempts to block that number
again in the Dialer, the migration workflow starts, completes, and
then the app crashes. This is because prior to migrating, the Dialer
doesn't know that the number is blocked in the framework, allowing it
to block the same number twice.

+ Since this case is specific to the situation where an already
blocked number initiates migration, this CL fixes the problem by
checking if the number is blocked in the framework, prior to blocking
it after the migration.

Change-Id: I31c8978afb871f364e63cab5cc6da3e5fd106b29
Fixes: 27720157