Fix broken transition to secure apps when relaunching

Previously, the logic for keeping occluded mode during transitions
would also trigger when transitioning from the lockscreen to a
SHOW_WHEN_LOCKED activity that was already on-screen, which would
then prematurely clear the occluded mode and break the launch
transition.

With the fix, the lockscreen must already be occluded before the
keep-occluded-during-transitions logic is triggered.

Bug: 21525170
Change-Id: I1d9ad95b2dd0728af7986f09adb67a617cc62cb2