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
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:124e3e5ebb1d1e7bcc3795f61728957dfb2a127f)
Merged-In: I16637a2c958c16120c0451ab742a4c2e04093e6c
Change-Id: I16637a2c958c16120c0451ab742a4c2e04093e6c