Give floating flag to previous activities in the stack

When launching stacked activities, only the last one is flagged as
floating in sysui when tapping halo.
This causes the system to show the launcher when the first stacked
activity is launched (this first intent gets the flag
FLAG_ACTIVITY_TASK_ON_HOME
in TaskStackBuilder).
This change makes all the stacked activities get the floating flag if
the last one has it.