Ignore orientation of finishing activity

There is a special case that the invisible activity can still
report orientation for who uses orientation_behind.
e.g.
 Top activity: orientation_behind
 The occluded invisible activity behind: orientation_sensor
Then it will pick sensor for top activity.

But if the top activity is already finishing, it should stop
to search the behind activities and keep current orientation.
Otherwise an intermediate orientation may be chosen which
may cause unnecessary orientation change.

The case may happen when launching orientation_behind activity
with clearTaskOnLaunch or intent flag CLEAR_TASK that will finish
the original activity at the same time.

Fix: 317680642
Test: ActivityRecordTests#testGetOrientation
Change-Id: I3c288ebd79cfe5b780d1f6427432d3c0bb0e568b