gui: PatternPassword: allow any N*N grid

Rather than only supporting a 3x3 grid, allow for multiple grid sizes
(using the CyanogenMod method of generating passphrases for non-3x3
grids). Also fix the detection of touches, as the old code was far too
sensitive for larger grids (and also didn't make much sense).

Change-Id: I343ef654e6d29ce0cb790a28281be7c7c9b171d9
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>