Exporting a superflat map with custom generation resets the generation to default

Release_466466

More info can be found here: https://bugs.mojang.com/browse/MCPE-25689

Summary:
After editing `FlatWorldLayers` inside level.dat, loading the world, and then exporting it again, the FlatWorldLayers gets reset.

Impact:
This could be a huge problem for partners when packaging up their map. I know quite a few partners (myself included) depend on this feature to produce "void worlds" to keep world file sizes down (not only keeping the filesize down, but it's a HUGE pain to generate a gigantic void world any other way which is necessary due to how large a player's render distance can be)

Repro Steps:

1. Generate 1.2 world in Minecraft
2. Edit world with NBT editor (for example MCCToolChest PE or MCEdit)
3. Change FlatWorldLayers to generate only 1 layer of void
4. Load up world in Minecraft (FlatWorldLayers should work)
5. Export world through Minecraft
6. Either check the world with a NBT editor, or load the world in game

Observed Results:
FlatWorldLayers reverts back to whatever Minecraft originally generated (1 layer bedrock, 2 layers dirt, 1 layer grass)

Expected Results:
FlatWorldLayers stays as edited.

Screenshots/videos attached:
n/a