libtaskmanager: consider current activity when computing first task index

For example, there are two activities. Two launchers are in one activity, and
3 launchers arein the other activity. Before this change the first task
index will be 5 because the index doesn't consider the current activity.
After this change the first task index will either be 2 or 3 depending
on the current activity.

CCBUG: 386055


(cherry picked from commit a67e3674)