[6/n] Pin ActivityStack

Avoid matching split-rules once TF is pinned, which including:

- Do not resolve split-rule while starting activities from the pinned
  TaskFragmentContainer.

- Do not remove the existing secondary container (clear-top) if the
  container is pinned because the pinned container won't be occluded
  by the new split.

- Do not launch placeholder if the container is pinned

Bug: 208573140
Test: atest PinActivityStackTests
Test: atest SplitControllerTest
Change-Id: I7a2d5d77ab87acefd3c2fd9ba11852e150adc59d