EdgeBackGestureHandler finds launcher properly

On headless user 0, the recents package (e.g. launcher) might not be
installed in user 0. The system should handle this better - we just need
to fetch the package, even if it isn't installed in user 0.

Test: on headless user 0 mode, make sure EdgeBackGestureHandler doesn't
crash
Bug: 216636343

Change-Id: Ib11c592cdb8c1941a24d5065453b97fea9df547b