Use std::array<char, 16> instead of uint128 for StatelessResetToken.

Using absl::uint128 does not make much sense since this is an opaque piece of
data with a fixed size of 16 octets (that happen to be the same as the size of
uint128), no numerical operations are performed on it.  Keep absl::uint128
within QuicUtils::GenerateStatelessResetToken(), necessary for the hash
computations, but use the new alias StatelessResetToken elsewhere.

This is a follow-up to a drive-by comment at cl/366984087.

PiperOrigin-RevId: 367245431
Change-Id: Ie79796e22ed4b7c6862a55e0280e7943378d559a