shimless: Update code usages of 'cancel' to 'exit'

* Change usages of 'cancel' to 'exit' in the main nav and the onboarding page
* Update tests to reflect changes
* Additional CLs will follow that will continue to refactor 'cancel' to 'exit' in the code

Bug: 1198187

Test: I tested this locally by running Chrome in my VM using the
      standalone shimless config.

Change-Id: I1965a00bf108f464cd9dfdac0cb659676bf2b510
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3647153
Reviewed-by: Gavin Williams <gavinwill@chromium.org>
Commit-Queue: Camden Bickel <cambickel@google.com>
Reviewed-by: Michael Checo <michaelcheco@google.com>
Cr-Commit-Position: refs/heads/main@{#1006736}