[2/n] Pin ActivityStack

Make TaskContainer#mContainers private. This is a preliminary
step of handling the pinning container in the following CLs.

Noted that the #getTaskFragmentContainers and #getSplitContainers
returns the real list to improve the efficiency, but we shouldn't
make modifications on the returned list directly.

Bug: 208573140
Test: atest SplitControllerTest TaskContainerTest
Change-Id: Ia5b298c402f287777c2cbdd4fb1bb23c8c5102b7