[macOS, Keyboard] Fix Khmer layout crashing

[dkwingsmt]

Fixes flutter/flutter#108729

The previous algorithm queried usLayoutGoalsByKeyCode without checking existence. std::map creates empty entries for these keys in this case, leading to 0 logical keys.

Also, the previous algorithm checked the entire 128 key codes, which included numpad keys. This allowed mandatory goals for digit keys to be fulfilled by numpad, which is not the intention.

Pre-launch Checklist

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[gspencergoog]