Contacts: Don't crash if Blocked Number Storage is disabled

If the provider is disabled or missing, trying to resolve
content://com.android.blockednumber/ will result in an
IllegalArgumentException. Don't allow to block numbers if the
provider is missing, or else the Contact app will crash upon
opening the preferences.

Solution based on commit 96f9c9cd2cfcde1a1c8dcc46ff21ed6452c27eb5
by Gabriele M <moto.falcon.git@gmail.com>

Change-Id: Ib95d5e7e8b4b625e6c20f537b84da9d09ebef6ea