Intent intent = new Intent(session, GeofenceTriggerReceiver.class);
        final PendingIntent proximityIntent = PendingIntent.getBroadcast(session.getApplicationContext(), 0, intent, PendingIntent.FLAG_NO_CREATE);

To briefly explain the structure of my app, as context for my questions:

MainActivity adds a ViewPagerFragment which contains a TabLayout and a ViewPager. This ViewPager sets a FragmentPagerAdapter that hosts two content fragments (AllPostsFragment and HomePostsFragment, but only the former is relevant right now). AllPostsFragment contains a RecyclerView and observes a LiveData in the ViewModel, in order to set PostsAdapter with the latest data for the RecyclerView. The LiveData in the ViewModel that AllPostsFragment observes, mirrors via switchMap a LiveData exposed by the Repository. It contains the deserialized JSON data for Reddit's front page... basically 25 content links and associated data. It's being fetched by Retrofit w/ Gson adapter.

1. First question is about the onItemClick callback in MainActivity. It's called by PostsAdapter whenever a row in the RecyclerView is clicked. What I want it to do is display a fragment (HelloWorldFragment for testing purposes...eventually it will be a detail view for the RecyclerView items). When I call getSupportFragmentManager on the activity as in line 46, everything works as expected. HelloWorldFragment appears and covers the screen.
...

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/view_pager_fragment_base_view"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".fragments.ViewPagerFragment">

    <android.support.design.widget.TabLayout
        android:id="@+id/tabs"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

    <android.support.v4.view.ViewPager
        android:id="@+id/pager"
...