Lockscreen targets: more unification

Don't hardcode pixel sizes, and use a shared method to derive drawable
inset.

Change-Id: I2adf9230c5aa8efadddd713c435b37af24e23115
Merge "Lockscreen targets: more unification" into cm-10.2
Merge branch 'cm-10.2' of https://github.com/CyanogenMod/android_frameworks_base into DJL10.2