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/preference/ToolsInstallerPreference.java

@@ -30,10 +30,6 @@ public class ToolsInstallerPreference extends Preference {
         toolsInstaller = applicationComponent.getToolsInstaller();
     }
 
-    public ToolsInstallerPreference(final Context context) {
-        this(context, null);
-    }
-
     @Override
     public CharSequence getSummary() {
         return getContext().getString(state.messageResourceId);