Consider making `GetAllocationSize` const in the engine

[dnfield]

When working on flutter/engine#18706 I found that GetAllocationSize was not const, when it probably could be - it should be an idempotent call, especially since the Dart runtime only expects to call it once, and it should never really have to modify any internal resources.

If it can be marked const, we can revert some of the changes to make pointers non-const introduced in flutter/engine#18706.

[github-actions]

This thread has been automatically locked since there has not been any recent activity after it was closed. If you are still experiencing a similar issue, please open a new bug, including the output of flutter doctor -v and a minimal reproduction of the issue.