Add dial string validation for FAC codes.

FAC codes are formatted as:
#FAC#MSISDN*

The existing implementation just checked for a matching known FAC code
in carrier config and then just skipped processing it as a USSD/MMI and
dialed it as is.

It turns out in some regions, carriers have FAC codes that mean something
completely different than the same code dialed as a UUSD.

So, *#112*6505551212# and #112#6505551212* do something different 🙄.

To fix this, as part of the validation for a FAC code, we'll also verify
that the original dial string matches the expected format for an FAC code.
This ensures the aforementioned cases get handled as expected; in the
formed case as a USSD/MMI, and in the latter case as an FAC code.

Test: Added new unit test cases for this.
Test: Ran unit tests.
Fixes: 330528194
Change-Id: I74a680f31f6a9f4704d8cf80df397fca4cb01d37