Migrate PackageManagerHelper to MainThreadInitializedObject

- This is in preparation for other things that can be loaded with
  PMH initialization and prevents duplicate temporary helpers from
  loading this many times.
- Most calls in PMH can use the app context, but one call requires
  starting activities/showing toasts so that one needs to take the
  context and can be made static instead.

Bug: 323112914
Test: atest NexusLauncherTests
Change-Id: Id11c780955880cf49c022cbf2744c41e1b696355

src/com/android/launcher3/dragndrop/AddItemActivity.java

@@ -162,7 +162,7 @@ public class AddItemActivity extends BaseActivity
             finish();
             return;
         }
-        ApplicationInfo info = new PackageManagerHelper(this)
+        ApplicationInfo info = PackageManagerHelper.INSTANCE.get(this)
                 .getApplicationInfo(targetApp.packageName, targetApp.user, 0);
         if (info == null) {
             finish();