[unified_analytics] avoid flutter_tools feature flags from getting truncated when sending to unified_analytics

Currently, the enabled features we send for analytics remains in the same order that they were enabled. So if we had 3 features:
- my-first-feature
- my-second-feature
- my-thrid-feature

The list will always be = `"my-first-feature,my-second-feature,my-thrid-feature"`

In GA4 we have a 36 character limit so if the string is longer than 36 characters, anything after 36 gets truncated and dropped. This could send a false signal that a given feature is not being used,

We should encode the feature names more compactly, to minimize the loss of data due to truncation