Move tasks to fullscreen stack when removing the pinned stack.

- When the user dismisses the PiP, it's not necessarily desirable
  for the tasks to be destroyed and non-accessible from Overview.
  As a result, we shift the tasks from the pinned stack to the
  fullscreen stack, either under the existing top visible
  fullscreen task, or to the top of the stack if the fullscreen
  stack is not visible.

Bug: 33304808
Test: android.server.cts.ActivityManagerPinnedStackTests
Test: #testRemovePipWithNoFullscreenStack
Test: #testRemovePipWithVisibleFullscreenStack
Test: #testRemovePipWithHiddenFullscreenStack

Change-Id: I802f44949705f86c55bdbd8ce0fd839513a8d95b