Fix broken transition to secure apps when relaunching

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