Reland "Revert "[App Management] Enable App Management by default.""

This is a reland of d44f4549

Original change's description:
> Revert "[App Management] Enable App Management by default."
>
> This reverts commit 2fd01309.
>
> Reason for revert: Disabling App Management before branch point.
>
> Original change's description:
> > [App Management] Enable App Management by default.
> >
> > Enable App Management by default. Current entry points are about:settings
> > and about:app-management.
> >
> > Bug: 906508
> > Change-Id: I0118a4b51eda6e65bc72186e771fc2ede2acb45f
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1610658
> > Reviewed-by: Dominick Ng <dominickn@chromium.org>
> > Commit-Queue: Jeevan Shikaram <jshikaram@chromium.org>
> > Cr-Commit-Position: refs/heads/master@{#659341}
>
> TBR=dominickn@chromium.org,jshikaram@chromium.org
>
> # Not skipping CQ checks because original CL landed > 1 day ago.
>
> Bug: 906508
> Change-Id: I302364e6573eaf21b7a0a3a93ecf517fc473740a
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1636652
> Commit-Queue: Jeevan Shikaram <jshikaram@chromium.org>
> Commit-Queue: Dominick Ng <dominickn@chromium.org>
> Auto-Submit: Jeevan Shikaram <jshikaram@chromium.org>
> Reviewed-by: Dominick Ng <dominickn@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#664595}

TBR=dominickn@chromium.org

Bug: 906508
Change-Id: Iae08c2de1231360e6882b74aeee5d02f4a7fae2f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1716045Reviewed-by: Jeevan Shikaram <jshikaram@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Commit-Queue: Jeevan Shikaram <jshikaram@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680335}

chrome/common/chrome_features.cc

@@ -665,7 +665,7 @@ const base::Feature kSystemWebApps{"SystemWebApps",
 
 // Enables or disables the App Management UI.
 const base::Feature kAppManagement{"AppManagement",
-                                   base::FEATURE_ENABLED_BY_DEFAULT};
+                                   base::FEATURE_DISABLED_BY_DEFAULT};
 
 // Disable downloads of unsafe file types over insecure transports if initiated
 // from a secure page