LockPatternUtils: add a static way to retreive pattern from string

The non-static method assumed that the pattern would be the same size as
the currently set length, but when we are setting up a new pattern, it
may have a different length.

Also add a utility method to compare two patterns.

Ref: CYNGNOS-661

Change-Id: I73d0e8d016c5c93ba64ab697579ac39d8b78716a
Signed-off-by: Roman Birg <roman@cyngn.com>