feat(android): option to add extra at the bottom of the keyboard to avoid accidentally exiting keyboard

**Describe the solution you'd like**

A proposed solution from [Ibrahima](https://community.software.sil.org/t/feature-request-on-keyman-developer/4606):

> I noticed that we usually tap on phone navigation bar mistakenly and exit application, which is annoying. That is happening because the Spacebar of the keyboard (the botton) is much too close to the navigation bar of the phone.
> Why not add some sufficient space at the bottom of the keyboard to avoid that?
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