Allow forcing a specific height with a environment variable

Evert Heylen requested to merge evert/squeekboard:height_env_var into master

See this discussion for some context. To recap, I'm developing a kiosk application and I have very little control over the hardware in kiosks and want to display larger touch targets than on a phone, so I'd really like a way to force a specific size.

Compared to the patch I sent in that discussion, I added a small test and defined the environment variable as a static constant instead of hardcoding it everywhere.

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