Recents: ClearAll: Fix glitch on quick double tap

To reproduce:
Open five or more apps, open recents and tap twice very quick on
clear all button.

Setup a boolean to control the thread starting, don't relaunch if
clearAll's run() has been executed and is already firing up childs.

Along with this https://gerrit.paranoidandroid.co/#/c/4346/ UI/UX should
look great no matter how crazy and how many times the user click on recents.

Signed-off-by: Carlo Savignano <stevewatersy@gmail.com>
Change-Id: I0d393838c23e2585222cf7b181c2f7a6385494cd
SystemUI: Iterate only over existing Pie snap points