Move AppWindow to extensions

Move AppWindow, AppWindowRegistry and AppsClient from apps/ to extensions/browser/app_window
Change the namespace from apps to extensions.
Fix GYP and DEPS.

BUG=403726
TBR=sky@chromium.org for include and namespace changes

Review URL: https://codereview.chromium.org/494033002

Cr-Commit-Position: refs/heads/master@{#292599}
117 files changed