Backup the activities to finish on TaskFragmentContainer exits

Also remove a few TODOs.
- Split rule tag could still be null for apps using earlier
  WM Jetpack library.
- No more plans to enable the feature internally as the flag
  enters next-food.

Bug: 289875940
Test: verified on sample app
Flag: com.android.window.flags.ae_back_stack_restore
Change-Id: I526ad7a97fd7f35a11367508a9c1bcc3984aef67