Fix bug that prevented waking from dream

If a device was securely locked, FLAG_TURN_SCREEN_ON failed to
wake the device from dreaming even if FLAG_SHOW_WHEN_LOCKED
was set. The fix allows SHOW_WHEN_LOCKED activities that are about
to turn the screen on to show even if they're not currently the top
most window with SHOW_WHEN_LOCKED set (which in this case would always
be the dream)

Bug: 21719374
Change-Id: I8d7bce05d95ed9de50b5a52b0973562b070aca5a