fix(storage, android): stream handler & event channel clean up on completion

[russellwheatley]

Description

The stream handler and event channel were never cleaned up once the StorageTask had finished, causing memory leak and ultimately an out-of-memory exception.

Test Setup

Using the following setup, I continued to create Storage Tasks.

ElevatedButton(
  onPressed: () async {
  Uint8List buffer = Uint8List(5000000);
  Reference ref =  FirebaseStorage.instance.ref();
  await ref.child('flutter-tests/putdata').putData(buffer);
},
child: const Text('Storage test'),
),

Profiling App before fix

I profiled the app to see the memory footprint.

Here is the baseline memory footprint starting the app, it starts with 269.1mb ( to the right of image, half way up):

I pressed the button 10 times which creates 10 Storage tasks, here is the lastest memory footprint which has now risen to 317mb:

Finally, you can see that eventChannels and streamHandlers are retaining 10 objects as anticipated:

Profiling App after fix

Profiled the app after pressing the button at least 10 times you can see that the memory remains similar to the baseline memory measured earlier:

Finally, looking at the size of eventChannels and streamHandlers, you can see they are at 0 as we would expect with a fix:

Related Issues

closes #13385
closes #13460

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• 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 ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change.
• No, this is not a breaking change.