Fix and simplify Screenshots UI check test

There's no need to flip the pref and poll for the page action to appear
or disappear. Instead, just directly enable or disable via the
AddonManager. This won't conflict with the pref observing code, because
that code only reacts to pref changes, not to addon state changes.

test/addon/browser.ini

@@ -1,6 +1,2 @@
-[DEFAULT]
-support-files =
-  head.js
-
 [browser_screenshots_ui_check.js]
 skip-if = os == 'win' && debug # Bug 1394967

test/addon/browser_screenshots_ui_check.js

@@ -1,5 +1,8 @@
 "use strict";
 
+ChromeUtils.defineModuleGetter(this, "AddonManager",
+                               "resource://gre/modules/AddonManager.jsm");
+
 const BUTTON_ID = "pageAction-panel-screenshots_mozilla_org";
 
 function checkElements(expectPresent, l) {
@@ -68,11 +71,15 @@ function promisePageActionViewChildrenVisible(panelViewNode) {
 }
 
 add_task(async function() {
-  await promiseScreenshotsEnabled();
-
-  registerCleanupFunction(async function() {
-    await promiseScreenshotsReset();
-  });
+  // If Screenshots was disabled, enable it just for this test.
+  const addon = await AddonManager.getAddonByID("screenshots@mozilla.org");
+  const isEnabled = addon.enabled;
+  if (!isEnabled) {
+    await addon.enable({allowSystemAddons: true});
+    registerCleanupFunction(async () => {
+      await addon.disable({allowSystemAddons: true});
+    });
+  }
 
   // Toggle the page action panel to get it to rebuild itself.  An actionable
   // page must be opened first.