Add TASK_LAUNCHING_BEHIND to change flags

When ActivityRecord#mLaunchTaskBehind is true, the newly-launched
task is positioned behind home. However, for compatibility reasons,
it's marked as visible, so there's no way for Shell to tell if a
newly launched task should be shown to the user or not.

This CL adds a transition change flag for this purpose.

Bug: 268178951
Bug: 268130984
Bug: 268131581
Test: atest StartActivityTests#testStartActivityTaskLaunchBehind
Change-Id: I3c5b517de5c56fe365cc9dd487e7d42dfa584513