llkd: Disable in userdebug builds by default

While llkd helps in discovering issues in apps which leave zombies, it
creates issues for dogfooders when apps are killed. Disable it by
default.

Bug: 202411543
Test: boot and check llkd not running
Test: `setprop ro.llk.enable true` enables llkd
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: If93bf9e981eaa3921a9da5f3160db26c4fe17e66