Fix memory leak in NestedScrollViewState.

[ksokolovskyi]

This PR fixes a memory leak in the NestedScrollViewState, which did not dispose created SliverOverlapAbsorberHandle.

Description

• Fixes Memory Leak: NestedScrollViewState does not dispose SliverOverlapAbsorberHandle. #135246.

Tests

• Adds a new test to nested_scroll_view_test.dart to verify that SliverOverlapAbsorberHandle dispatches memory events;
• Updates nested_scroll_view_test.dart to use testWidgetsWithLeakTracking.

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 [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.

[ksokolovskyi]

cc @polina-c

[polina-c]

Google testing failed because it did not catch up with conversion of maybeDispatchObjectCreation to static:

ERROR: third_party/dart/flutter/lib/src/widgets/nested_scroll_view.dart

1627: ChangeNotifier.maybeDispatchObjectCreation(this); Instance member 'maybeDispatchObjectCreation' can't be accessed using static access.
                                                        Too many positional arguments: 0 expected, but 1 found.
                                                        #static_access_to_instance_member #extra_positional_arguments


google3:///[third_party/dart/flutter/lib/src/widgets/nested_scroll_view.dart:1627](https://cs.corp.google.com/piper///depot/google3/third_party/dart/flutter/lib/src/widgets/nested_scroll_view.dart?l=1627&ws=tap-presubmit-server/107738340&snapshot=5):22: Error: Member not found: 'ChangeNotifier.maybeDispatchObjectCreation'.
      ChangeNotifier.maybeDispatchObjectCreation(this);
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^

[ksokolovskyi]

@polina-c thanks for the review!