Lockscreen Targets : Fix empty magnetic behaviour

When no valid targets are set, set settings to null in order
to enable magnetic targets (aosp behavior)

Change-Id: I0ccaad25cc85f55b99e4c34cb4ffe2d06bda4a80
Fix adding apps to lockscreen targets without explicit icon selection.

Change-Id: I28c5a2fb72aabd892cc861d592ef20bcbf9fb891