Use activity-exit animation for translucent-only close

This is parity w/ legacy. Many translucent tasks are designed
with the expectation of sliding-down for closing animation.

Bug: 282655010
Test: launch a translucent task (in 3 button nav) and press
      home button.
Change-Id: I9b04f5534742a9b853d870cff48a1f4eda4d97f2