SystemUI: avoid lock screen shortcut picker crash

Fixes crash when picking lock screen shortcut app, then rotating, then
hitting the back button.

Ticket: FEIJ-299

Change-Id: I728052878ca3fc87243878ef96acea2e052aed6d
Signed-off-by: Roman Birg <roman@cyngn.com>
(cherry picked from commit d8a30fc9)