Update Apps Foundation flags.

These features have had their launch dates pushed back. Update the flags
accordingly.

Change-Id: I9507c636a615661645bc0e8a77aab5c4ca187ba4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1743234Reviewed-by: Jeevan Shikaram <jshikaram@chromium.org>
Commit-Queue: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685360}

chrome/browser/flag-metadata.json

@@ -110,12 +110,12 @@
   {
     "name": "app-management",
     "owners": [ "//chrome/browser/resources/app_management/OWNERS" ],
-    "expiry_milestone": 76
+    "expiry_milestone": 79
   },
   {
     "name": "app-service-ash",
     "owners": [ "//chrome/services/app_service/OWNERS" ],
-    "expiry_milestone": 76
+    "expiry_milestone": 79
   },
   {
     "name": "arc-available-for-child",