Revert "[WIP] Multi-window"

This reverts commit 690306ac111f415efd2031a21d424b7305962b47.

Conflicts:
	core/java/android/view/IWindowManager.aidl
	packages/SystemUI/src/com/android/systemui/recent/RecentsPanelView.java
	services/java/com/android/server/am/ActivityStack.java
	services/java/com/android/server/wm/WindowManagerService.java