Lockscreen Shortcut : Delete entry if reset

Delete the entry when resetAll is selected so that original AOSP
behaviour is present, such as magneticTargets.

Thanks Eddy Witkamp for reporting

Change-Id: I04e03336d8f51fd9683f6d859a649a37e68655ba
Settings: Lockscreen Targets Icon Revamp

Adjust the size of custom app icons and resize and round the corners
of gallery icons for a more uniform look.

Before http://imgur.com/cTOqdwX

After http://imgur.com/EKAeFiw

Change-Id: I75f3656ae5b4ac4a4bd1c1cbed6c07ab2863f4fd
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