Move the task to back while pressing back on the last activity

... of the secondary (or the primary) container.

The primary container was removed along with the secondary container
while the last Activity in the secondary container was finished by
back event. Therefore, starting window will be shown if starting the
app again from Launcher.

However, the last Activity in the secondary container should be
considered as a relative task root if the Activity is displayed
adjacently or being a companion with the task root activity.
The task should be moved to back vs. finishes the Activities to
speed up the launch next time, like commit 2dec42fa does.

Bug: 240669850
Test: launch Settings
Test: atest WindowOrganizerTests
Change-Id: I13b33afb8602f63062f6b33742453c3e93fdfc20