Quick cleanup across the board

- Clean up imports
- Adjust method visibility
- Drop in NonNull annotations where needed
- Prevent potential NPEs from nulled getActivity calls
- Remove unused methods

Signed-off-by: Harsh Shandilya <me@msfjarvis.dev>

app/src/main/java/com/wireguard/android/fragment/TunnelDetailFragment.java

@@ -1,6 +1,7 @@
 package com.wireguard.android.fragment;
 
 import android.os.Bundle;
+import android.support.annotation.NonNull;
 import android.view.LayoutInflater;
 import android.view.Menu;
 import android.view.MenuInflater;
@@ -33,7 +34,7 @@ public class TunnelDetailFragment extends BaseFragment {
     }
 
     @Override
-    public View onCreateView(final LayoutInflater inflater, final ViewGroup container,
+    public View onCreateView(@NonNull final LayoutInflater inflater, final ViewGroup container,
                              final Bundle savedInstanceState) {
         super.onCreateView(inflater, container, savedInstanceState);
         binding = TunnelDetailFragmentBinding.inflate(inflater, container, false);