Adds a visual indicator of where the autoclick scroll will occur.

See go/cros-dwell-ux for mocks. Still waiting for spec but expect
that it will not change substantially.

Also defaults autoclick scroll to the center of the active display.

Bug: 947068
Change-Id: Ia49a88ac045b540fc3828b008f06ed2818db1114
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1648804
Reviewed-by: Anastasia Helfinstein <anastasi@google.com>
Commit-Queue: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#668139}

• ash/BUILD.gn[diff]
• ash/autoclick/autoclick_controller.cc[diff]
• ash/autoclick/autoclick_controller.h[diff]
• ash/autoclick/autoclick_ring_handler.cc[diff]
• ash/autoclick/autoclick_scroll_position_handler.cc[Added - diff]
• ash/autoclick/autoclick_scroll_position_handler.h[Added - diff]
• ash/autoclick/autoclick_unittest.cc[diff]