Use 99th percentile as the maximum frequency when adjusting icon color

When adjusting the icon color according to frequency, compute the 99th
percentile of all measured frequencies and use it as the "true" maximum
frequency. This is required because in a small percentage of cases the
current CPU frequency reported by Linux can be unrealistic, for example
4.59 GHz on a CPU that has an actual maximum frequency of about 4.4 GHz.
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