Release PluginBinding instance when activity is detached.

[mvanbeusekom]

The plugin keeps hold of the PluginBinding instance once an Activity is attached, but never releases the instance when the Activity is detached from the plugin. This will cause the Activity to be leaked (as described in issue #1256). This PR will release the PluginBinding instance when the onDetachedFromActivity method is called, making sure the handle to the Activity is released, which prevents leaking the Activity.

Fixes #1259

Pre-launch Checklist

• I made sure the project builds.
• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is does not need version changes.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I rebased onto main.
• I added new tests to check the change I am making, or this PR does not need tests.
• I made sure all existing and new tests are passing.
• I ran dart format . and committed any changes.
• I ran flutter analyze and fixed any errors.

[TimHoogstrate]

LGTM