Only bind fast scrollbar when attached if not already bound.

This was overriding the previously bound RecyclerView if multiple
were attached simultaneously. Instead, the appropriate container
(All Apps / Widgets) should bind the active RecyclerView whenever
it changes, with the onAttach serving as a fallback to ensure
the scrollbar has an initial RV to avoid NPEs.

Fix: 234591523
Bug: 235476489
Test: Manually checked All Apps from Launcher and Taskbar, as well
as Widget bottom sheet. Also ran relevant Tapl tests.

Change-Id: I06e27d2f66f9778087711a566817b6806ec7218b