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}

chrome/common/chrome_features.cc

@@ -617,7 +617,7 @@ 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