Do not consider finishing dream activity as occluded

Otherwise it causes the next activity to be visible with
redundant lifecycles. And if screen orientation is changed,
the next activity may get wrong configuration by performing
relayout with intermediate visibility.

Test: 1. Enable dream, off/on screen and wait screen timeout.
      2. Rotate device, touch dream activity and unlock keyguard.
      Check that home activity shows with correct layout.
Bug: 181837363
Change-Id: I46aac2f2ca4208de39ace98fc8bc3917d13880ac