Remove possibility of zero chars from passwords

Remove possibility of zero chars from passwords

scrypt pads the password with zeros. Our patterns use 0 to represent
the top left dot. So patterns that end there are equivalent to ones
that end one short.

After much thought, the best solution is to change the way we
represent patterns in keyguard. This, however, is a big change.

The short term solution is to change the pattern representation in vold
so that we are storing the correct thing. Later we will change keyguard
to handle patterns correctly and remove quite a few hacks from vold
(use of hex, this code). b/17840293 created to track this.

Bug: 17751714
Change-Id: I30cdffb0f0db406d2e2b6c54d4153d120d975318