add setImmersiveFullscreen to toggle navbar in android

[vanadium23]

Hello, koreader team.

I really miss navbar on my android tablet, and want to add back the feature to toggle full screen.

This is first part, just to add proper function, second one will be done within original repo.

Original issue https://github.com/koreader/koreader/issues/8532.

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This change is 

[pazos]

Hey, @vanadium23, thanks for your time!

Looks good. Could you please send a PR against the main repo before this gets merged?. I would like to see the whole picture and do some quick tests

[vanadium23]

@pazos add pull request to koreader: https://github.com/koreader/koreader/pull/9485