Convert ScrollCaptureViewHelper#requestScroll to async

This is a prerequisite for supporting longer scrolling intervals
to allow scrolling in steps, allowing the main thread to loop in
between.

This also provides an opportunity for a ScrollCaptureViewHelper
instance to delay until content stabilizes (layout, image loading,
etc).

Bug: 195109744
Test: atest ListViewCaptureHelperTest ScrollViewCaptureHelperTest \
      RecyclerViewCaptureHelperTest WebViewCaptureHelperTest
Change-Id: Ia7c146338472b958e85992f39941ec844115d66f