Stop passing UserLeaving around

Activity#onUserLeaveHint() did not call on the split-secondary top
activity while starts home activity via gesture-nav in splits, so
the app did not enter pip.

Home task was moved to top and #resumeTop was called after the
recents animation finished. It was no-op since home activity was
already be resumed, but the TaskSupervisor#mUserLeaving was consumed
and was reset. The previous top activity was paused (made invisible)
later during activity visibilites update, without userLeaving hint
though.

This CL reset the TaskSupervisor#mUserLeaving when the requested
operation completed and stop passing the state over methods.


Bug: 176989982
Test: atest ActivityLifecycleLegacySplitScreenTests
Change-Id: Iaa6a7525d632a536f8a3faa6814a2a1b838e6830