Lockscreen targets: more unification

Use shared method to derive drawable inset and make the rounded
corners for icon files also display when adding the target initially.

Change-Id: I98ea3e522c4c6240f08e11007d53509a3d2c15f8
Merge "Lockscreen targets: more unification" into cm-10.2
Merge pull request #6 from CyanogenMod/cm-10.2

Cm 10.2