Change Linkify leniency to VALID

The linkify logic used POSSIBLE as its leniency setting, which resulted
in false positives such as 4-digit years being interpreted as phone
numbers. Changing to VALID as per recommendation of libphonenumber
people, which fixes this problem.

Bug: 18489494
Change-Id: I77d330285de46de2fdda22daed41392106ec6ddd