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/util/AsyncWorker.java

@@ -22,7 +22,7 @@ public class AsyncWorker {
     private final Handler handler;
 
     @Inject
-    public AsyncWorker(final Executor executor, @ApplicationHandler final Handler handler) {
+    AsyncWorker(final Executor executor, @ApplicationHandler final Handler handler) {
         this.executor = executor;
         this.handler = handler;
     }