Fix checking for invalid numbers.

Some of the checking for invalid numbers was broken by code
cleanups yesterday, and blocking of numbers which can't be normalized
had been allowed (which is no good). Fixing this, by unifying checks
for whether or not a number can be blocked.

Bug: 25343277
Change-Id: Ia631a3e53e3f9ed58e173055a5e18086d95c9d02