Fix CrostiniAppModelBuilderTest.EnableAndDisableCrostini

1. unregister CrostiniAppModelBuilder as observer in destructor
2. don't create an extra CrostiniAppModelBuilder in the test

Bug: 939528
Change-Id: I45a62445495e2b6c6ed284cd257c38318b682303
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1509792
Reviewed-by: Timothy Loh <timloh@chromium.org>
Reviewed-by: Nicholas Verne <nverne@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638849}
2 files changed