Revert of Merge apps/pref* to extensions/browser/pref* (patchset #1 of https://codere...

Revert of Merge apps/pref* to extensions/browser/pref* (patchset #1 of https://codereview.chromium.org/493003002/)

Reason for revert:
Breaks GN:

http://build.chromium.org/p/chromium.linux/builders/Linux%20GN/builds/10608

Original issue's description:
> Merge apps/pref* to extensions/browser/pref*
>
> The pref's exclusive user AppWindow will move to extensions/browser
>
> Merge apps/pref_names.{cc,h} to extensions/browser/pref_names.{cc,h}
> Merge apps/prefs.{cc,h} to extensions/browser/extension_prefs.cc
>
> BUG=403726
> TBR=bartfab@chromium.org for include fix in configuration_policy_handler_list_factory.cc.
>
> Committed: https://chromium.googlesource.com/chromium/src/+/3da4417c5baa6b8fe0be9aa012f0fd3294d9dbc7

TBR=benwells@chromium.org,bauerb@chromium.org,bartfab@chromium.org,hashimoto@chromium.org
NOTREECHECKS=true
NOTRY=true
BUG=403726

Review URL: https://codereview.chromium.org/499253002

Cr-Commit-Position: refs/heads/master@{#291652}