Shift focus when a task with multiple activities gets removed

When ActivityTaskManager#removeTask is called for a task with multiple
activities in a freeform environment, no activity in other tasks gets
resumed, the corresponding transition doesn't run, and it times out.

This doesn't happen in fullscreen environments becaue the tasks behind
get visible when the top task is removed and one of them gets resumed on
another code path.

If a task has only one activity in a freeform environment, when the top
activity is removed, there's no activity left, so
adjustFocusToNextFocusableTask() is called in finishIfPossible() and the
corresponding app transition will be eventually run.

However, if there are multiple activities, when the top activity is
removed, currently finishIfPossible() believes that there are some other
activities left in the task although they will be removed just after
this.

This CL fixes this by reverse the order of removing activities in
ActivityTaskManager#removeTask(). This way, at the point the top
activity is removed, all the other activities have been gone and
adjustFocusToNextFocusableTask() is called properly.

Bug: 249658397
Test: WM CTS
Change-Id: I769208a7c90a8e82e4bb4c4168f19bca3cf3507e