mdpi: Override only the required dimensions

We should only override whatever dimension is needed
Should fix the lockscreen targets icon size changed
here: http://review.cyanogenmod.org/31987

Change-Id: I602e8d3c2b501a2b8df56a1855c9279b67a767f8
Keyguard: Fix PIN layout on sw320dp-mdpi devices

Fix a long standing lockscreen layout problem that was introduced in 4.2
Also cleanup the dimensions overrides to match cm-11.0

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

Change-Id: Ia3cdc0d10217a0a2504a7287f480d9f036e66eee