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
Merge "mdpi: Override only the required dimensions" into cm-10.2
Merge branch 'cm-10.2' into martincz