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/model/ShortcutsChangedTask.java

@@ -78,7 +78,7 @@ public class ShortcutsChangedTask extends BaseModelUpdateTask {
 
         if (!matchingWorkspaceItems.isEmpty()) {
             if (mShortcuts.isEmpty()) {
-                PackageManagerHelper packageManagerHelper = new PackageManagerHelper(
+                PackageManagerHelper packageManagerHelper = PackageManagerHelper.INSTANCE.get(
                         app.getContext());
                 // Verify that the app is indeed installed.
                 if (!packageManagerHelper.isAppInstalled(mPackageName, mUser)