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/model/Tunnel.java

@@ -37,8 +37,8 @@ public class Tunnel extends BaseObservable implements Keyed<String> {
         this.state = state;
     }
 
-    public static boolean isNameValid(@NonNull final CharSequence name) {
-        return NAME_PATTERN.matcher(name).matches();
+    public static boolean isNameInvalid(@NonNull final CharSequence name) {
+        return !NAME_PATTERN.matcher(name).matches();
     }
 
     public CompletionStage<Void> delete() {