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: Ie58ddc81b4ea897b4243404a58fae47583e49e1a

Conflicts:
	packages/SystemUI/src/com/android/systemui/recent/RecentsPanelView.java
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
Remove unwanted imports and fix derps

Conflicts:
	packages/SystemUI/AndroidManifest.xml
	packages/SystemUI/src/com/android/systemui/recent/RecentsPanelView.java
	packages/SystemUI/src/com/android/systemui/statusbar/phone/PhoneStatusBar.java
	packages/SystemUI/src/com/android/systemui/statusbar/phone/QuickSettingsController.java