Revert "Change Linkify leniency to VALID"

This reverts commit 9dfe86d4.

That change moved the lenience from POSSIBLE to VALID, which eliminated
false positive links, especially 4 digit phone numbers, but caused
significant false negatives, leading to CTS test failures
(android.text.util.cts.LinkifyTest#testAddLinks7 in particular).

The true fix requires new functionality to validate phone numbers in
a mobile context. In the meantime, the best solution is to revert.

Bug: 18708556