[cr145][Android] Remove deprecated FirstRunFragment.onNativeInitialized

Chromium changes:
https://chromium.googlesource.com/chromium/src/+/6ba08476e9338abfe074dbf92fb2ac01c9583bf3

commit 6ba08476e9338abfe074dbf92fb2ac01c9583bf3
Author: Boris Sazonov <bsazonov@chromium.org>
Date:   Wed Dec 17 07:13:35 2025 -0800

    [Android] Remove deprecated FirstRunFragment.onNativeInitialized

    No longer needed.

    Fixed: 40232440
    Change-Id: I147403115e8b928e8088ea437dbefa8d444da636
    Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7262576
    Commit-Queue: Tanmoy Mollik <triploblastic@google.com>
    Auto-Submit: Boris Sazonov <bsazonov@chromium.org>
    Reviewed-by: Tanmoy Mollik <triploblastic@google.com>
    Cr-Commit-Position: refs/heads/main@{#1559945}

browser/brave_ads/android/java/org/chromium/chrome/browser/onboarding/BraveAdsOnboardingFragment.java

@@ -32,7 +32,6 @@ public class BraveAdsOnboardingFragment extends Fragment implements FirstRunFrag
 
     private int mProgress;
     private static final int END_TIME_SECONDS = 3;
-    private boolean mNativeInitialized;
 
     private LottieAnimationView mAnimatedView;
 
@@ -62,6 +61,8 @@ public class BraveAdsOnboardingFragment extends Fragment implements FirstRunFrag
 
         setActions();
 
+        startCountdown();
+
         return root;
     }
 
@@ -80,14 +81,6 @@ public class BraveAdsOnboardingFragment extends Fragment implements FirstRunFrag
         }
     }
 
-    @Override
-    public void onNativeInitialized() {
-        assert !mNativeInitialized;
-
-        mNativeInitialized = true;
-        startCountdown();
-    }
-
     @Override
     public void setInitialA11yFocus() {
         if (mTvTitle == null) return;