Refactor phone context parsing for RFC3966 numbers.

[silvio2402]

• Refactor the logic to extract the phone context from an RFC3966 URI to a new PhoneContextParser class.
• Refactor the number normalization methods normalizeDigits and normalizeDigitsOnly to a new PhoneNumberNormalizer class.

  This keeps PhoneNumberUtil.normalizeDigitsOnly as a shell because it is public.

• Move the constants that are common between PhoneContextParser and PhoneNumberUtil to a new Constants class.

This is a refactor and should not change the logic of the parser.

Context: b/370922064

[silvio2402]

Refer to new PR: #3791