Call MobileAds.initialize on background thread

[alihassan143]

Description

This PR changes the initialization of the Android MobileAds SDK to run on a background thread. This is recommended by the Google Ads team to reduce ANRs. See https://ads-developers.googleblog.com/2024/07/reduce-anrs-when-implementing-mobile-ads.html

Related Issues

*#1135

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
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Breaking Change

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• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[LTPhantom]

Thanks for your Pull Request. Unfortunately, it seems there are some format problems before any other check can be run.

[alihassan143]

@LTPhantom
dart pub global run flutter_plugin_tools format
after that now code is formatted

[alihassan143]

@LTPhantom kindly allow workflow to run

[alihassan143]

@LTPhantom all test passed kindly review the pr

[malandr2]

There is more to this PR than just calling MobileAds.initialize on the background thread, such as adding the OPTIMIZE_AD_LOADING flag to our samples. Those are unrelated changes, can you please remove all extra changes except for the code change in FlutterMobileAdsWrapper.java?