[WIP] Multi-window

[[[ USE PS 18 IF YOU WANT TO TEST - WORK IN PROGRESS ]]]

This is a big work in progress, which currently has some bugs,
but begins to look okay.

You can already multitask a little bit, by using the
app switcher and long-pressing an icon, then select "Add to
Split View".
Last window touched gets the key focus.

Code is very ugly, I know. Don't merge it, don't use it
for now. Unless you want to make your code ugly.

Enjoy :)

Change-Id: I4a03a84539fd3ac1273f0d3503fe8fafcd16be26

Conflicts:
	core/java/android/app/Activity.java
	core/java/android/app/IActivityManager.java
	core/java/android/view/IWindowManager.aidl
	packages/SystemUI/res/values/custom_strings.xml
	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