Cleaned up wording around "valid" and "formattable".

We don't actually parition by "formattable", we parition by "valid". An invalid number like 456-7890 can be formatted to E164 ("+14567890") but what ParitionedNumbers actually does is parition by valid/invalid (and then converts the valid numbers to E164).

Also added a new sharded test suite for phonenumberproto tests which had occasionally been timing out on TAP.

Test: existing
PiperOrigin-RevId: 181800443
Change-Id: Id64fc32c893025b0115dd350dd87e3277607f21c