Move flag for home launching from activity to task.

The variable ActivityRecord.mLaunchHomeTaskNext was used to indicate
that the home task should be launched when the activity completed.
This only mattered when it was at the end of a task. As the activity
launched other activities within the same task it needed to be
migrated from activity to activity and task to task. This became
too complicated and was at the wrong level to begin with.

By moving the flag to TaskRecord.mOnTopOfHome the logic is simpler
and the results more predictable.

Fixes bug 10602256.

Change-Id: If0b752522b77be9918f1dba221d0ff670fc01af8