Make MobileBert tokenizers independent from Bert

[501Good]

What does this PR do?

Copied the code from Bert tokenizers into MobileBert tokenizers to make the latter self-contained.

Fixes #19303

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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Who can review?

@sgugger

[sgugger]

Just one last nit and we will be good to merge! Thanks a lot!