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)

Workaround it by hiding clear all recents button.

Change-Id: I23d10edfd336041739fe80352ee01987e671dbd7