Re-land "Make AndroidView participate in gesture arenas." (#21097)

This re-lands commit c594696.

The original PR was breaking the integration test as while the
gesture was active the AndroidView's parent widget got rebuilt which
disposed the _AndroidViewGestureRecognizer and dropped pointer events
from that point on.

This change adds a unit test that reproduces the failure case, and
fixes it by not rebuilding the _AndroidViewGestureRecognizer if the
list of gesture recognizers did not change.