Recents:  add missing TypedArray.recycle() call

Change-Id: Ie8ee595ff2b8c499f9e20fdf2a072ef114ee33dc
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