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 branch 'cm-10.2' of https://github.com/CyanogenMod/android_packages_apps_Settings into DJL10.2