[Android/IME] Translate composition events (beforeInput with insertCompositionText/insertText) to delete events

[Reinmar]

Potentially a part of #12425.

When in composition on Android (i.e. using e.g. GBoard, even in English mode), pressing Backspace triggers beforeInput with insertCompositionText with a non-empty target range that replaces the entire word.

A more optimal way to modify the model would be if we translated that to a delete event so only the bit that needs to be removed was touched in the model.

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