[camera] Ensure setExposureOffset returns new value on Android

[mvanbeusekom]

Since version 0.9.0 the Android setExposureOffset method doesn't return the newly configured offset after updating the exposure offset. This will cause a Unhandled Exception: Null check operator used on a null value error in the camera_platform_interface as it does expect a result.

This PR fixes the problem by correctly returning the newly configured exposure offset on Android.

Resolves flutter/flutter#89281

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

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[google-cla]

☹️ Sorry, but only Googlers may change the label cla: yes.

[BeMacized]

LGTM. Waiting for the tree to turn green