SystemUI: prevent NPE on clear recents double press

Pressing Clear all button twice fast or while the list
is clearing but not yet fully cleared causes sysui to crash due to
mRecentTaskDescriptions being null: (http://pastebin.com/2gRFvf8k)

Change-Id: I23d10edfd336041739fe80352ee01987e671dbd7
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