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