Don't verify the scrollbar drawable

Avoids infinite invalidations caused by re-use of scrollbar drawable
during a single draw() pass. Does not address the general problem of
drawable reuse causing unnecessary invalidations as a result of calls
to setBounds() invoking invalidateSelf().

Bug: 26533725
Change-Id: I99e9c2dfe4ddfc833569e40e7268dcb03e931fc9