Ensure transparent assistant works with PiP

- When an always-on-top stack is visible, then ensure that a stack is
  visible if it is behind a translucent assistant stack (similar to
  b/37294521).

Bug: 63055581
Test: android.server.cts.ActivityManagerPinnedStackTests
Test: android.server.cts.ActivityManagerAssistantStackTests
Test: #testPinnedStackWithAssistant
Change-Id: I4ffec7bbcbd11c496604fcd1da381fd10fb6a892