Fix derp observer

If per app color is disabled, pie gravity ain't work?

Change-Id: Ia1c1cfc23b521863db7993cad2581b38ea011f0a
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.