/fly toggle does not persist after switching dimensions

If you toggle /fly (to be true) in for instance the nether, when you enter the overworld it becomes false/setting does not persist.

Start by locating the /fly command and the code that handles switching between dimensions. Reproduce the issue by enabling /fly in the nether and entering the overworld, then trace where the setting is stored. Done means the enabled state persists across dimension changes and the behavior is covered by a regression test.

Written by the indexing model from the issue text.