Deprecate MemoryAllocations in favor of FlutterMemoryAllocations

Disposables in pure Dart projects can't use MemoryAllocations in Flutter. So, to be leak-trackable they need a Dart-only class. MemoryAllocations in Flutter is renamed to make the name available to a non-Flutter, Dart project.

Before:

if (kFlutterMemoryAllocationsEnabled) {
  MemoryAllocations.instance.dispatchObjectCreated(
    library: 'package:flutter/gestures.dart',
    className: '$MultiDragPointerState',
    object: this,
  );
}

After:

if (kFlutterMemoryAllocationsEnabled) {
  FlutterMemoryAllocations.instance.dispatchObjectCreated(
    library: 'package:flutter/gestures.dart',
    className: '$MultiDragPointerState',
    object: this,
  );
}

Landed in version: 3.19.0-2.0.pre
Landed in stable: 3.22.0

Relevant issues:

• Rename MemoryAllocations to FlutterMemoryAllocations (Issue 140622)