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
Floating window: Fix possible NPE

Avoid possible NPE if app provides no interface such as pushbullet's
test notification & other apps which provide one.

Change-Id: I14a0d41de36aaced76c928dc1f22c8a7f3913917
Merge pull request #2 from CarbonDev/kk

update