Fix unnecessary and harmful task movement.

Changing the focus to a new activity should not move it to the top of
the task stack. When the previous activity fully pauses and the new
focused activity resumes then it will be brought to the top of the
task stack at the proper time. Moving it there prematurely causes the
ActivityManager and WindowManager stack sequences to be out of sync.

Fixes bug 9518153.

Also remove false warnings in validateTopActivitiesLocked() and add
debug for task movement to TaskStack.

Change-Id: Ib57500b07ded97223155cda7ef603aecc9b642c3