Add static patternToString() with pattern size

Sometimes we need to generate a pattern string when lock pattern utils
cannot query the actual size of the pattern. Add a method which doesn't
depend on LockPatternUtils and accepts a pattern length parameter to
generate the string.

Change-Id: If73334b7030800bed2eec0cd0934b762b10dcaa3
Signed-off-by: Roman Birg <roman@cyngn.com>