taskswitcher: polish gesture animations

some animations don't look great currently:

• flick to homescreen makes task switcher look like it fully opens before closing for a split second (and doesn't feel smooth in movement)
• quick switch diagonal flick gesture animation is super quick, could probably be slowed down a bit (currently shares animation with opening a specific task which I think has the right duration so that needs to be split)
• not directly animation but the task should probably not follow the finger up all the way on the screen and become more reluctant to following the further up the user goes

and maybe others as well?