overlay: writeback: Do not create writeback instance on dumpsys

Do not create a writeback instance during dumpsys.
Instead use the dumpsys to return empty if no instance exists.
This fixes the issue where
a) Unnecessary instance is created.
b) Dumpsys has garbage, if a valid instance didn't exist

Change-Id: Ic8ea5ddd8d05203234da7c7dfccd2c2b497e62f7
3 files changed