Multi-window support

Change-Id: Ie5860c2105ad9c971157b41d65f8fab337461ef6

Conflicts:

	core/java/android/app/IActivityManager.java
	services/java/com/android/server/am/ActivityStack.java

Resolved conflicts:
	core/java/android/app/IActivityManager.java
	services/java/com/android/server/am/ActivityStack.java