Fix for launch app in floating mode from recents

Change-Id: I216b02a60cd46b7fef49e04fe090169c4703bcb6
Keep app when opening floating window from recents panel

Start activity asynchronously by appending it to system's
message queue to let RecentsPanelView cleanly execute
dismissAndGoBack before the floating window gets started.

Change-Id: Ie9c74b9f0b39f3aee81c62246cdcae15de83ee0d
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