Fix ScrollViewFunctionalTest running on Test thread

Fixes: 299470923

ScrollViewFunctionalTest was executing methods that should only
be run on the UI thread on the Test thread. This CL moves those
calls to the UI thread.

This also reduces the chance of flakiness on very slow devices.

Test: run repeatedly on forrest
Change-Id: I16637a2c958c16120c0451ab742a4c2e04093e6c