Move some logic of removeTask() into TaskRecord.

Follow up from 891146c6 to move some
logic into TaskRecord.

It removes seemingly unnecessary onActivityRemovedFromStack(), and also
encapsulates mLRUActivities into ActivityStack itself by moving stack
dump logic into ActivityStack.

Bug: 119259346
Test: Unit tests pass. go/wm-smoke. dumpsys activity activities doesn't
change.
Change-Id: Ic76fe64afd048a4fb489fd76125d072d4432da07