Fix header style for feed and follow management activities.

When we changed to use an AppBar, it inhereited the default styles
(blue background) which is not what we want.  Instead, we want a
style more like the settings activity.

Bug: b:197975174
Change-Id: Id1de32509fb449a361eafedfa38e2afe92627145
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3135232


Reviewed-by: Cathy Li <chili@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Commit-Queue: Peter Williamson <petewil@chromium.org>
Cr-Commit-Position: refs/heads/main@{#917746}