Hiding add number button in Blocked Management UI

+ When new filtering is possible, we should hide the add number
button from the blocked numbers management UI.
+ Added method to FilteredNumberCompat to check if it's possible to
use the new filtering implementation. This is needed because prior
to migrating to new filtering, users need to be able to unblock
numbers. Just checking the SDK version is not sufficient, we need to
know if the user has migrated their numbers.

Bug=26664600

Change-Id: I60433465074911f13a26736221ddacc9a8bbcf88