Lockscreen targets: more unification

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

Change-Id: I2adf9230c5aa8efadddd713c435b37af24e23115

Conflicts:
	core/java/com/android/internal/util/cm/LockscreenTargetUtils.java
	policy/src/com/android/internal/policy/impl/keyguard/KeyguardSelectorView.java