ProfileActivity: Replace fragments instead of hiding

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

app/src/main/java/com/wireguard/android/PlaceholderFragment.java

@@ -0,0 +1,18 @@
+package com.wireguard.android;
+
+import android.app.Fragment;
+import android.os.Bundle;
+import android.view.LayoutInflater;
+import android.view.View;
+import android.view.ViewGroup;
+
+/**
+ * Fragment that shows a placeholder message.
+ */
+
+public class PlaceholderFragment extends Fragment {
+    @Override
+    public View onCreateView(LayoutInflater inflater, ViewGroup parent, Bundle savedInstanceState) {
+        return inflater.inflate(R.layout.placeholder_fragment, parent, false);
+    }
+}