Kivy can't get the keyboard height with SDL2

[inclement]

This is partly a Kivy issue (it needs some code to check this), but SDL2 doesn't expose the keyboard height. Probably it will be necessary to add code to the java bootstrap, as was done in the pygame bootstrap.

SDL2 does have an option to mark a textinput location, whereupon it will handle the keyboard shifting itself, but (although we could expose this) it isn't as flexible as kivy's existing options.

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