Create superclass for BlockedNumberAdapter.

To facilitate reusings the same views and adapter for both the
manage blocked numbers list and the import list. The two will
use different cursors because they are pulling data from different
places (Contacts db vs FilteredNumbers db), and it seemed better
to reuse code rather than creating parallel implementations.

Bug: 23351616
Change-Id: I5cac5f2a6b8b3bf4a6e8f54c8bc3f1c1500ac5e7