Keyguard: Fix PIN layout on sw320dp-mdpi devices

Fix a long standing lockscreen layout problem that was introduced in 4.2

Before: http://i.imgur.com/9sVKHRR.png
After: http://i.imgur.com/DLY4DRu.png

Change-Id: Ia3cdc0d10217a0a2504a7287f480d9f036e66eee