public RequestManager get(FragmentActivity activity) {
        if (Util.isOnBackgroundThread()) {
            return get(activity.getApplicationContext());
        } else {
            assertNotDestroyed(activity);
            FragmentManager fm = activity.getSupportFragmentManager();
            return supportFragmentGet(activity, fm);
        }
    }
Retrieves and returns the RequestManagerRetriever singleton.
core/src/main/java/com/bumptech/glide/manager/RequestManagerRetriever.java