Turn off smooth scrollbar

The dev docs say, "should be set to false if your adapter will display
items of varying heights." Since ComposeMessageActivity shows items of
varying heights, this was causing the scrollbar to grow and shrink while
scrolling. With this change, the scrollbar doesn't scroll smoothly, but
the thumb now accurately reflects the scroll position. Bug 2709895.

Change-Id: I1130a1001b093adc11b85c38c095fdc3792b4c1a
Merge commit 'cb286204489066916c0a512ec9254bc07e129c79' into oxygen_1005

* commit 'cb286204489066916c0a512ec9254bc07e129c79':
  Turn off smooth scrollbar