ActivityTrigger:  New class to invoke when activity starts/resumes

Change-Id: Ib57aa4d3767e27b5bba658b27e084fc3ef042c36

Conflicts:
	core/jni/AndroidRuntime.cpp
	services/java/com/android/server/am/ActivityStack.java

Conflicts:
	core/jni/AndroidRuntime.cpp
	services/java/com/android/server/am/ActivityStack.java