fix(banner): prevent NullPointerException from mAdView reassignment race

[GingerAdonis]

Problem

The banner crashes with a NullPointerException when banners are shown/removed in quick succession (common for apps that drive a banner from route changes):

java.lang.NullPointerException: Attempt to invoke virtual method
'void com.google.android.gms.ads.AdView.setAdUnitId(java.lang.String)' on a null object reference
  at com.getcapacitor.community.admob.helpers.AdViewIdHelper.assignIdToAdView(AdViewIdHelper.java:28)
  at com.getcapacitor.community.admob.banner.BannerExecutor.lambda$createNewAdView$5(BannerExecutor.java:245)
  at android.os.Handler.handleCallback(Handler.java:1027)
  at android.os.Looper.loop(Looper.java:412)
  at android.app.ActivityThread.main(ActivityThread.java:9998)

Root cause

BannerExecutor holds the banner in a single mutable field, mAdView. Work posted to the UI thread (createNewAdView, updateExistingAdView) and the AdListener callbacks all read/write the field at execution time, rather than the AdView instance they were created for.

Capacitor dispatches plugin calls on a single background thread, so showBanner/removeBanner bodies never overlap — but the GMA SDK's ad callbacks fire independently on the UI thread. That opens a race:

1. showBanner creates adViewA, calls loadAd(), attaches a listener.
2. removeBanner runs → adViewA.destroy(), mAdView = null. adViewA's load is still in flight.
3. showBanner runs again → mAdView = adViewB, posts createNewAdView's UI task.
4. adViewA's load now fails → onAdFailedToLoad fires for adViewA's listener. Its if (mAdView != null) check sees adViewB, so it destroys adViewB and nulls the field.
5. createNewAdView's task finally runs → AdViewIdHelper.assignIdToAdView(mAdView, …) with mAdView == null → setAdUnitId NPE.

The same shared-field hazard lets updateExistingAdView NPE on mAdView.loadAd(...).

Fix

Bind each posted UI task and the AdListener to the AdView instance they belong to (a captured final local) instead of the shared field:

• createNewAdView / updateExistingAdView capture the AdView and return early if it is no longer the current mAdView when the UI task runs.
• The AdListener callbacks return early when their AdView is no longer current, so a stale onAdFailedToLoad / onAdLoaded from an already-removed banner can no longer tear down — or emit events for — the current banner.

Behavior note: a stale onAdFailedToLoad no longer emits SizeChanged(0,0) / FailedToLoad. Those events described a banner the JS layer had already discarded, so emitting them was itself spurious.

Testing

This is a threading-correctness change; the race depends on GMA SDK callback timing and isn't practically reproducible in a unit test. Verified by reasoning through Capacitor's single bridge-thread + UI-thread callback model. The file is unchanged by prettier --plugin=prettier-plugin-java; cd android && ./gradlew clean build to confirm compilation.