Bug 1875502 - Create standardized update init flow r=nalexander,settings-reviewers,application-update-reviewers

A number of changes are made as part of this patch:
 - A consistent way of initializing update is created. This is automatically run when methods that need it are invoked.
 - The "post-update-processing" notification has been removed. Post update processing is now done through the new `nsIApplicationUpdateService.init` or `nsIApplicationUpdateService.internal.postUpdateProcessing`.
 - Post update processing no longer waits for the sessionstore-windows-restored observer service notification
 - Post update processing is no longer invoked only when the `update.status` file exists. It is now run unconditionally.
 - Explicitly initialize before we potentially clean up updates from the update UI.

Note that the update service stub and a few consumers of it ought to be able to wait for post update processing to be done but they currently do not. That will be done later in this patch stack when we rework the stub.

Differential Revision: https://phabricator.services.mozilla.com/D209126

browser/components/preferences/main.js

@@ -2549,6 +2549,9 @@ var gMainPane = {
     let um = Cc["@mozilla.org/updates/update-manager;1"].getService(
       Ci.nsIUpdateManager
     );
+    // We don't want to see an idle state just because the updater hasn't
+    // initialized yet.
+    await aus.init();
     if (aus.currentState == Ci.nsIApplicationUpdateService.STATE_IDLE) {
       return;
     }