Add APIs to remove tasks.

You can remove sub-tasks inside of a task, or an entire task.

When removing an entire task, you can have its process killed
as well.

When the process is killed, any running services will get an
onTaskRemoved() callback for them to do cleanup before their
process is killed (and the service possibly restarted).

Or they can set a new android:stopWithTask attribute to just
have the service automatically (cleanly) stopped at this point.

Change-Id: I1891bc2da006fa53b99c52f9040f1145650e6808