[Settings] Add a method to finish the current fragment

Some settings page need to finish when the user is done with them.
Previously we used getActivity().onBackPressed() to perform this, but
we might want to handle finish and back gesture differently, and also it
might have unintended interaction with metrics.

This patch introduces a canonical method for settings fragments to
declare that they are finished and the settings activity should show the
previous fragment.

We added the new method to SettingsLauncher because existing methods are
somewhat similar to the new one (handling navigation). In a later change
we plan to rename it to reflect the new addition.

Bug: b/356743945
Change-Id: I368e1edd155ef6443e5b81903380189eca8ca4d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5886984
Reviewed-by: Theresa Sullivan <twellington@chromium.org>
Commit-Queue: Shuhei Takahashi <nya@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1359686}