Fix call to get the current user id from a secondary SystemUI process.

- The secondary SystemUI process's KeyguardUpdateMonitor is not updated
  so we can't rely on it to get the current user id.

Bug: 38372598
Test: Switch users, launch some tasks, and switch back. Ensure that original
      tasks still exist

Change-Id: I26a7c70b5f7032a106dc1342c8b366e4797e8fa0