HTMLForm: Clean up 0 handling

PHP, particularly with in_array, really has problems with integer 0
versus non-numeric strings. Let's clean that up by converting values to
strings more agressively and using the $strict option to in_array.

Oddly enough, the one place where strict in_array was being used already
broke when If4175332 stringified the values in one array but not the
other.

Bug: 62268
Change-Id: Id34e654eb8d0e70d093b11445273e542e491e265
(cherry picked from commit 5f57d5d657b7bb4b656b99acad1b2e85b9c834a8)