Fix 6021938: Improved target support in lock screen

This adds the ability to enable or disable target icons based on the drawable
resource of the target.

It also fixes a bug where we'd show the camera while displaying
the PIN/PUK unlock screen or when it's disabled by DevicePolicyAdmin.

Minor simplification and cleanup KeyguardUpdateMonitor callbacks.

Change-Id: I33fad56a2203bc8b7bcd0300c20478711a56713a