Insert home task into history when sending to back.

- There is an assumption that moving a task to the back moves it
behind the home activity. This fix ensures that the home activity
will be launched by some activity before the task that has been
moved to the back.

- Convert "private final" declarations to private only.

- Convert container tests for size()==0 to isEmpty().

- Remove unused variable.

Fixes bug 10228536.

Change-Id: I51741ee7a09652e27afffcc50c14953bf0aa27ea