Do not prefetching accessibility nodes while a view is scrolling

Accessibility framework default enable prefetching accessibility
nodes in application, when an accessibility service is trying to
query a node on the screen. These nodes are stored in the
AccessibilityCache which is in the accessibility service to save
the IPC between accessibility service and application on screen.

This mechanism does not help performance in some scenarios. For
example, an application with thousand nodes on screen and user
needs to scroll the content frequently. In this case, the
AccessibilityCache will be cleared frequently, and the prefetching
behavior also happens frequently. It becomes an extra effort and
drop the performance.

In this patch, we stop prefetching for a little time while a
scrolling event is detected. This can mitigate performance drop
symptom while user is scrolling.

Bug: 143730839
Test: Manual
Change-Id: I336ee2907ba63c32c6e53259589817dfd14b99e1
Merged-in: I336ee2907ba63c32c6e53259589817dfd14b99e1
(cherry picked from commit 1be88099)